So I found this rather interesting SCMP article from a Reddit post that asserts, almost offhand, that
ships built in Guangdong and Fujian were based as much on Southeast Asian traditions as on Chinese rivercraft.
and also mentions a few other things like Javanese junks that towered over Portuguese ships and shrugged off cannonballs.
This is interesting. I'll admit upfront that I know little about the Javanese except that they were a large and powerful Southeast Asian empire with several tributary states.
Were their ships really the basis for the stereotypical Chinese junk we think of today and is the descriptions given of them resisting Portuguese cannon really true?
TL;WR: The article is correct.
Warning: Lots of ship-related jargon.
I'll start with a quote from Diego de Couto, a Portuguese chronicler of the early 17th century:
[The Javanese] are all men very experienced in the art of navigation, to the point that they claim to be the most ancient of all [sailors]... It is certain that they formerly navigated to the Cape of Good Hope, and were in communication with the east coast of the Island of San Laurenzo [Madagascar].
As Couto realized, the Javanese, and by extension insular Southeast Asian, maritime enterprise was an immense undertaking and one which did indeed influence Chinese shipbuilding.
Were their ships really the basis for the stereotypical Chinese junk we think of today
China has a long and distinguished history of building ships, especially for the massive rivers of the empire. Considering that the current definition of 'junk' refers to virtually all Chinese (or generally East Asian) ships, it's inaccurate to say that Chinese ships as a whole originated from Java/Southeast Asia. We should note the clear contrasts between the 'typical' Chinese ship (e.g. the one you might have found in the Yellow Sea) and the 'typical' Javanese/Malay ship. To quote Anthony Reid (see sources below)
[For the Javanese ship], its hull was formed by joining planks to the keel and then to each other by wooden dowels, without using either a frame (except for subsequent reinforcement), nor any iron bolts or nails. The vessel was similarly pointed at both ends, and carried two oar-like rudders and a lanteen-rigged sail. It differed markedly from the copiously described "Chinese" type of vessel, which had its hull fastened by strakes and iron nails to a frame and to structurally essential bulkheads which divided the cargo space. The Chinese vessels had a single rudder on a transom stern, and except in Fujian and [Guangdong] they had flat bottoms without keels.
Although the 'typical' Chinese and Javanese/Malay ships shared a few traits, such as the multiple sheathing of the hull (that is, superimposed layers of wooden planks), there were greater differences, especially the fact that the latter seldom used iron nails or axial rudders. So no, the Chinese ship and the Javanese ship were markedly different - in theory. But as the last sentence of the passage I quoted ("except in Fujian and Guangdong") suggest, the southern provinces of the Chinese empire saw significant Southeast Asian influence.
Marine archaeology has yielded a number of ships under the South China Sea, dating from the thirteenth to the seventeenth century. Although these ships were often built in China and manned by Chinese, they incorporate both Southeast Asian and Chinese influences. The defining features of these ships are (Manguin, 1993):
their planks are always fastened by iron nails to the frames, but they may also be dowelled together by wooden pegs; some have a single, axial rudder while others have quarter rudders; their holds are separated by bulkheads, but these are not structurally essential and kept watertight as in the Chinese tradition (all have waterways with limber holes hollowed out of the bulkheads); all their hulls are V-shaped and have a keel that plays an essential structural role, a striking difference with the traditional flat-bottomed, keel-less (Northern) Chinese build.
It was these hybrid ships - with both iron nails and wooden dowels, with both bulkheads and keels - that handled maritime trade in Guangzhou and Fujian, and between these two provinces and Southeast Asia. So yes, "ships built in Guangdong and Fujian were based as much on Southeast Asian traditions as on Chinese rivercraft" is a fair statement.
I would, however, note that just as Southeast Asian traditions influenced Chinese oceangoing ships, some elements of Chinese vessels influenced Javanese ships. One Javanese epic poem states that the King of Sunda (in West Java) in the 14th century traveled on a ship "such as was made in the land of the Tartars," or Yuan China.
is the descriptions given of them resisting Portuguese cannon really true?
Well, they are certainly corroborated by the Portuguese. The author of the article you linked is probably referencing this account by Gaspar Correia, a 16th-century Portuguese writer,^1 discussing a battle between one enormous Javanese ship and a Portuguese fleet (emphasis mine):
Seeing that the junco wanted to start fighting, the Governor got close to her with his whole fleet. The galleys started shooting at her, but this did not affect her at the least, and she went on sailing [...] The Portuguese ships then shot at her masts [...] and she dropped her sails. Because she was very tall [...] our people did not dare board her and **our firing did not hurt her at all, for she had four superimposed layers of planks, and our biggest cannon would not penetrate more than two.**^2
The average ship in Malay and Javanese waters appears to have had a tonnage of 350~500 metric tons. The Flor de la Mar, a famous Portuguese admiral ship, only had a tonnage of 400 tons. One Javanese ship, built in 1513, carried a thousand people aboard and may have approached 1,000 metric tons in tonnage! As for height, one Malay ship's deck was higher than the aftercastle of Flor de la Mar. So yes, very tall.
^1 There are others who noted that Javanese ships resisted cannon, but Correia's account is the most detailed.
^2 Some historians challenge such accounts and argue that the Portuguese embellished and exaggerated Southeast Asian ships to glorify their victories; generally, however, Southeast Asian historians believe the Portuguese.