i think the preimage is only revealed to the nodes across the payment route. i'm not aware of a site that lets you validate preimages similar to mempool.space, but

sha256(preimage) == payment_hash

i think the point nostr:npub1t0nyg64g5vwprva52wlcmt7fkdr07v5dr7s35raq9g0xgc0k4xcsedjgqv is trying to make is that nostr:npub1cashappn03s3cl2ljsdntv0v28e2um5lgx4vjctqjt23pcwzjhsqmtdg5l users don't operate the node they're spending from and aren't provided either of the values above which can be used to prove a payment was made.

contrast this with nostr:npub1xnf02f60r9v0e5kty33a404dm79zr7z2eepyrk5gsq3m7pwvsz2sazlpr5 connected to my own lightning node - i'm actually presented with both whenever i successfully pay any lightning invoice. i ignore them and go on about my day, but if the recipient ever disputed that i paid then, i could pull these up as evidence.