There are other threads discussing this already, some where even US Amex card holders transferred to an "authorized" / "registered" partner or so and KF did an audit and froze the account and such.

Just because Amex T&Cs in the US allow for this, doesn't mean KF T&Cs do and the receiving program (KF) matters here. I'd caution against doing it, but that is a personal decision.

There are folks that will have done this successfully a few times over and nothing happened and there are others that had a heck of a hard time getting this sorted and their account unlocked (usually the MR points where sent back to Amex or so). It takes a lot of time and there is no good way to talk to the team investigating. A bit like Hotel Loyalty Program fraud teams.

If OP has a way to get the MR points to his Amex account before transferring to KF, all is good, but see, strangely Amex does not seem to be allowing that, but permits it apparently when people transfer to a partner.

​​​​​​So check the KF T&Cs and decide for yourself, what Amex writes in their T&Cs doesn't matter, though it would be nice if they could be in synch. YMMV.