How did the ukrainian culture develop after the Kiever Rus? Why did people claim ( and still do ) that the ukrainian culture and ethnicity are made-up/fictional?

As a matter of fact I got my degree as a Ukrainian historian. Unfortunately I haven't used it for ten years so my description could be sketchy. Leave a comment if you see something wrong.

As you probably know Kievan Rus was federation of several Slavic tribes. It appeared on a trade route that connected Viking states and Byzantium between Baltic and Black seas and essentially was a birthplace of Belorussian, Russian and Ukrainian cultures. The golden period of the state was in the 11 century. But as age of the Vikings came to an end and Byzantium lost most of it's power, Kievan Rus lost influence as well. The state fell with Golden Horde invasion in the midst of the 13 century.

Mongols were waging wars with light cavalry — small horses, small people, small bows — which was amazing in steppe but not that great in the European forest. So their expansion stopped roughly on the territory of modern Ukraine. Kiev was destroyed. Most of the east slavic cities conquered. A few left though and amongst all Moscow, whose rulers sworn allegiance to Golden Horde.

There were also few others — northern states as well. Baltic principalities which formed Grand Duchy of Lithuania to join forces with Poland and become a power in Slavic lands.

So this how it went. Lithuanian-polish state to the north and the west. Golden Horde to the east and the south. And emerging Moscow duchery to the north and the east of them.

Territory of modern Ukraine became giant borderland. Steppe without king, godless veld between three giant Catholic, Islamic and Orthodox states. Even the modern name shows it. One of the explanation for the origin of the word "Ukraine" connects it with Old Russian "оукраина" — Borderland.

It was a fertile land though. So as time went Slavic people were settling here. Around 15 century Ukrainian language was formed sprouting from Old Russian language with strong influence of Polish. In the same time people started to organize armed militia opposing constant Islamic attacks. Later this militia was famed as cossaks.

In the seventeen century the cossaks of Polish lands rebelled and founded Zaporizhzhia, first Ukranian-speaking state. As it was settled by descendants of Kievan Rus (who were orthodox people), leaders of Zaporizhzhia decided to align with Moscow state. But Moscow czars didn't understand the concept of protectorate and in the following years declared independence of Zaporizhzhia slowly deteriorated. This in turn led to rebellion in the 18 century when cossaks leader Ivan Mazepa switched sides in Russian Swedish war.

The conflict between Moscow rule and Ukranian speaking people resulted in emergence of Ukrainain nationalism. In 18 and 19 century several cultural groups formed in city bourgeoisie. They were printing books in Ukrainian language. To answer this in 1887 Russian czar banned printing of all Ukrainian books.

I'm leaving a lot of cool details here. To name a few: reading my text you could think that the nation is formed around its language. But it more than this. Ukranians used eastern weapons, speaking with Polish melodic and managed to come up with one of the first democratic constitutions in history.

Finally: to answer your question why some people speak about Ukranian culture as made up.

Two things led to that.

The late development of national identity. There wasn't a lot of big cities in "Borderlands" (Kiev was destroyed and became big city only in the end of the 19 century). People were speaking one language, sharing one culture but they didn't had any cultural centers.
Russian resistance. Modern Ukraine is giant country with a lot of natural resources. 45 million people are living there. If you'll manage to convince some of them that they are not Ukrainians but misguided Russians you'll get influence and in the future their land for free. The first mentioned factor is helping this.

The fact that Ukrainians didn't have a lasting state in centuries not working well for them either. But I have an impression that people identity is strong, that they still keep the tradition of regular rebellions and as a nation become more and more solid.

Hope it helps!