But it does not apply to international transfers: Apparently international payments are no covered: https://www.financial-ombudsman.org.uk/businesses/complaints...

Of course the victims might have transferred to an account the scammer has in the UK

Falling for a company that doesn't deliver the level of quality it promised seems to stretch the definition of "scam".

Is there any requirement of diligence, or can you basically jump on any "100% profit in 3 weeks" scam and get (most of) the money back when it doesn't work out?

No, but in this case I think it does meet the definition of scam.

There is a requirement for diligence, but this requires not being "grossly negligent". Even this is waived for people classed as vulnerable (I do not know exactly what that means).