The FAQ[1] asks for 6 weeks before an event, so I suppose that's fair. That still seems like poor planning, though -- clearly the event was planned the the presumption that the funding would be approved. What would this open letter have looked like if the ask had been turned down altogether?
IMO the order of operations should be 1) ask for funding, 2) plan the event, not the other way around.
6 months is way in advance. PSF policy is to ask 4-6 weeks before the event (Timeframe: We require that applications be submitted 6 weeks before the event/project start date - this gives us enough time to thoroughly review, ask questions, and have enough time to send you the funds.), so 6 months is more than enough time.
And we're missing the issue: the organizers were asking for $9k, which by all means should have been an easy yes/no. If they would have asked for 100k, maybe, but $9k is chump change.
The funding FAQ gives general guidance of $2500 at the high end and still discusses virtual events. I suspect some of the problem is the process is for very low budget meetups, and virtual things, not six-day real world events.
IMO the order of operations should be 1) ask for funding, 2) plan the event, not the other way around.
[1] https://www.python.org/psf/grants/faq/