(I'm a co-founder of Balanced, a payments company also building push to card (p2c) functionality [1])
> There's no way to map debit card numbers to routing/account numbers (afaik).
You're correct.
> They're probably using the ATM network primarily and another method if it's not supported.
Based on the settlement times advertised, it looks like Stripe is simply performing "unreferenced refunds" to debit cards, rather than using the various ATM networks to push funds out to cards. If they were using the ATM networks, you could expect sub-10 min settlement times.
[1] https://www.balancedpayments.com/push-to-card