Not quite; after reaching Platinum by first flying 580XP, the next 300XP earned would go to requalification for Platinum; and the next 300XP earned after that would go to rollover for the next year. It is only then - after acquiring a total of 100(Silver)+180(Gold)+300(Platinum)+300(requal for Platinum)+300(max rollover)=1180XP from a standing start - that "multiples of 300XP" would come into play for converting to PfL years; a minimum of 1480XP would have had to be flown just to get a reduction from 10 years to 9 years waiting time....
Of course, requalifying for Plat, and having a max rollover of 300, ultimately will count as a "PfL year" - but not immediately - and I get the impression the OP wanted to have it sewn-up immediately as part of the transition - or at least make a severe dent in what is otherwise a long 10 year wait (with the uncertainties of what may change as regards the rules during those 10 long years).
One wonders why you didn't just pay €399 for a status match to Platinum last year, given that you have status with both BA and UA and given that Poland was part of the paid status match offer for most of last year (and, if I am not mistaken, for the first few days of 2025 as well) - €399 is a small price to pay to avoid the need to acquire 580XP just to reach Platinum in the first place!
(But even then, if you had been matched to Platinum at some time before 1 November 2024, you still would have had to have flown at least 900XP in order to reduce your waiting time for PfL by a single year from 10 years to 9 years...that's a hell of a lot of flying...)
Paid Status Match
(note: any additional queries about the status match, if any, should be posted in that thread rather than in this one)