If I understand it right, the Saliba loophole didn't work for Sarr because Sarr is from Senegal and thus has a Senegalese passport. Saliba could slink through as registered at both Woolwich and whichever club in France that he was loaned to because of his French passport. It is some sort of Europe vs not Europe loophole.
So on paper that should work for Luka, assuming that the EU/UEFA hasn't rectified the loophole since.
You may well be correct, but I think there was also a date problem as Sarr signed for Spurs on 27 August 2021 (after that season had started) and with his year of birth being 2002 it was not possible to get the 3 full year/season requirement to be club trained in before he became 21.
On a similar note we signed Will Lankshear on 31 August (after that season had started) but fortunately signed him young enough such that not being able to include his first part year (due to signing after season started) is recoverable from.
Its surprising how slipping dates by a week or two can sometimes cause issues which are not realised until too late.