The newcomer restriction (aka FGO original) allows both Illya and Kuro to participate, despite the fact that they both aren't original to FGO.
Probably only the people who created this event could tell you for sure, but I assume that it's because Kaleid Liner Prisma Illya is not considered to be a canon fate series.
I think that "newcomers" basically means that the servants didn't originate from an actual, canon, Fate media. Kaleid Liner is basically a professional fanfic. It's a third party spinoff that Type Moon had nothing to do with.
Kaleid and the other fate series are canon in Nasuverse. It is a parallel world, same with FGO. Fate series isn't just FSN world.
As for why Illya, Shiki, Kiritsugu considered newcomers, maybe because this is the first time they appeared as servants.
Kaleid was not written/produced/distributed by Nasu or Typemoon. Nasu has clearly drawn a line between his own universe, and Kaleid, multiple times in interviews. He has even literally called Kaleid a fanfic before. So if Nasu doesn't consider it to be canon, then it's not canon. It follows a completely different set of rules than the other worlds in Nasuverse (with the cards and the magical girls and such), so it's a stretch to call it a parallel world.
But you are correct that Illya/Chloe/Kiritsugu/Shiki aren't actually servants in any other media, so it would be safe to consider them FGO exclusive servants.
That might be the case (Nasu saying it's not related with Nasuverse). But those two being as Servants in F/GO - which is in fact part of the Nasuverse - still connects this outside work with it.
There even was an event only for Kaleid!
I don't think Illya counted as one in that show. Irisviel (Dress of Heaven) isn't one which is weird.
For the same reason that Medusa and Ibaraki are considered Heroes of Justice.
