I have to agree, I think there is more to this story than meets the eye as far as the OP is co concerned. Even if he was not completely open (or honest) about what happened that resulted in him receiving €8000, the damage to the reputation of Fixedfloat has already occurred and is unlikely to be repaired.
Having said all that, it is not impossible for the OP to receive a large payment for making the website but it is as though some pieces of the jigsaw were deliberately withheld in the OP. Now both the OP and Fixedfloat are silent but whereas nobody knows the OP, it is Fixedfloat that lost much more as a result of them seizing the funds. The KYC they put the client of the OP through is one of the most bizarre things I have read regarding exchanges asking for KYC.
Even if they've refunded OP and get into a resolution off-screen, or [if we assume for a while here] OP did not tell the full story and there are more to tell, I think the taint on their reputation is already done and can't be erased. They've show that they overcomplicate things, requesting way too intrusive KYC despite their claim of supporting privacy, and --the most important-- no one can guess whether their fund will get through or get confiscated when dealing with their platform, given they have their own blocklist and taint measurement.