As far as I know, at the time of "the Conquest", Hungarians were the largest monolithic culture in the region with around 300k people sharing the same culture and language (more or less). Also, the Carpathian Basin was mostly forests and did not really have much population (feel free to correct me if I'm wrong).

So, not a lot of "magyarisation" happened. Moravians to the north were pushed north, germanic attempts to invade were pushed back. Croatia was occupied and belonged to the crown for quite a while but it was never integrated into the county system and not a lot of Hungarians went to live there.

Also, nationality as an identifying factor was non-existent until the 19th century. Religion and feudal fiefdom were the prominent factors, and only nobility counted as members of "the nation". So "magyarisation" as such was not a thing (or any other kind of nationalisation).

In the 19th century, when nationalism became an ideology, there were attempts to "magyarise" other ethnic groups living in the kingdom but that didn't really go well and was probably the main cause of the drastic change of borders after WWI.

the Hungarian population is composed mainly of Slavs, genetically (as far as I know)

Eh, true and not true. There are typical Hungarian traits that can be identified in a number of people (facial structure similar to Finns etc.) Also, there are some genetic traits that are different here than in Europe (and are more common in Asia).

Also don't forget that the 16-17th centuries were a time of population boom across Europe. During this time, Hungary was under a 150-year Ottoman occupation and the population before and after was approx. the same. This means that the population growth happened, but was also killed off by the occupiers. So, after the liberation, surrounding nationalities came in to fill the gaps and this contributed a lot to more mixing.