Yeah, I've also come across some pretty creative solutions to terminating oversize wiring in the past as well. Clearly if the wire won't fit in the hole then it will be contrary to the manufacturers specifications for that product to terminate it.
I can't tell the dimensions of the neutral bar from the photo but if it's 7x9mm then the max current is usually 60 or 70 Amps, 8x12mm is normally limited to 100A (from memory) so if the wiring is 50mm˛ then there's also a good chance the neutral bar is also undersized for the OCPD in the DB.
My take on it is it's not compliant as it is but if you can find a 50mm pin lug or maybe a palm lug that will fit in the hole without butchering it first and if the neutral bar is current rated to same or greater than the main breaker then you might be in with a shout.
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