The Portuguese did not create the kingdom of Kongo. The Kongo kingdom, as well as neighboring polities like Vungu, Ngoyo and Kakongo to the north (in Congo-Brazzaville and Gabon) and to the south there we a number of Ambundu states already existing in 1485 when the Portuguese show up.

To answer your question, yes, the Kongo people and their neighbors already knew how to work iron in the 2nd century BC. Archaeology work done by James Denbow on the coast of Gabon found iron artifacts and smelting furnace carbon dated to circa 30 BC +/- 80 years.^1 Now, admittedly that is approximately 500-600 kilometers north of Mbanza Kongo. However, Jan Vansina gives us an overview of Namibia and Angola in How Societies Are Formed where he says that metallurgy (iron and copper) appears in the region perhaps late in the first half of the first millenium AD (i.e. 400-500 AD) but certainly by 600 AD.

The situation for agriculture is similar. Finds of charred, cultivated pearl millet in southern Cameroon carbon date to circa 400 BC, and Vansina gives dates closer to 250-150 BC in Angola.

So, by 1400s AD, Kongolese and their neighbors had long traditions of working copper, iron, and agriculture.

As to what the Portuguese changed. It is a bit unclear whether or not Kongolese and their neighbors understood the casting of bronzes and brasses. But, we can say with high confidence that the Portuguese introduced knowledge of how to cast large bronze and brass objects such as church bells.

Ditto, while the Kongolese had the capability to forge machetes, axes, arrowheads, iron bells; there is not evidence for the forging of something like the breastplate armor the Portuguese had, nor the construction of firearms and artillery. John Thornton discusses the military implications of those differences in Warfare in Atlantic Africa; 1500-1800.

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1 The Archaeology and Ethnography of Central Africa by Jim Denbow. pp 117

2 How Societies are Born by Jan Vansina. pp 33-40.