The allegations in the tweet don't make any sense, Substack doesn't charge the 30% fee on IAP and your subscriptions being on iPhone doesn't lock you into
Substack's ecosystem.
It is very strange though, because two months ago they were calling the "Epic injunction" that forced Apple to allow apps to use their own payment services a "big win for independent media". Now, for exactly the same service as before, everyone gets 30% less income or has to charge consumers ~45% more to cover the IAP fees and since it's all centralized under Substack's account the 15%-rate for unpopular stuff won't apply even though the individual authors may be unpopular.
That injunction is being appealed but is likely to remain in effect until next year at the earliest, so there's no particular pressure from that front to prefer the fee-heavy IAP over their own payments.
https://9to5mac.com/2025/06/04/substack-ceo-says-app-store-c...
