There isn't the same pressure on founders to raise a seed round as they are likely to exit YC with significant capital and runway. Seed investors still have capital allocation targets and are likely to improve their offers to encourage founders to accept investment.
This also incentivizes founders to raise at a higher cap to minimize the dilution from the $375k. I imagine it'll be harder for investors to negotiate a lower cap or get a discount.
I wouldn't call 400-500k a significant runway, but it should have some effect, since you can put of fund raising for at least a few months, maybe even 2 years if you're frugal. At that time you are hopefully further along.
put *off
They can wait longer before seeking funding. And since the terms for the 375k are based off the first funding round it makes sense to wait until the last second to minimize the percentage that goes to yc.