1. Because it isn't. Jotaro's giving an example of the current time dilation, he isn't saying Pucci is only that fast (he clearly isn't even in the fight itself, Pucci later dodges a time dilated bullet just fine). Just that if someone was 10mps, they'd now be "this" fast. And counting, due to the exponential increase.
If Pucci himself was, for example, like sub-rel before (say off Jolyne deflecting meteorites, Whitesnake slow-moing and blitzing gunfire, etc.), he'd be FTL after the dilation, and Made in Heaven's speed itself is already exceptionally fast even at a minimum.
And 2. Because Jotaro got ultra omega nerfed, the coma gutted his stats, he shouldn't even be able to walk, the recent magazine even points out that E in sustainability for Plat in that chapter is due to the coma, he can barely even manifest his Stand.
And 3. Part 6 was rewritten last second, the whole final fight is subject to random bullshit.
But honestly the fact Star Platinum is stated FTL via pure speed multiple times would make it obvious he's FTL either way. Even if we ignore the actual reasons and nuance to this, this isn't even a FTL anti-thing, it'd be a supersonic anti-feat, which is so laughably untrue that even humoring it at face value is absurd.