Hi Tómas!

> why is this node a part of Keleti, but not this area?

You're right, I haven't noticed that entry signals are already on the map. Therefore I guessed their positions. This is why the linked node was incorrectly part of the station.

> Would you agree that the borders extend too far in this case

Yes—thanks for the heads-up. I've adjusted the station boundaries accordingly: https://www.openstreetmap.org/changeset/150006448

> how would you justify the choice in where to map the boundaries of the station

The area of a railway station is between its entry signals on both sides (or, in this case, the only side). See: https://wiki.openstreetmap.org/wiki/Tag:railway%3Dstation#As_an_area