It's rare, but you're wrong here! I've seen fake transactions on Blockchain.info (now Blockchain.com) years ago, they were used by scammers to pretend they sent funds. Those fake txids didn't show up on any other block explorer.
That sounds like a double-spent transaction that still lingers in their database.
Or TXIDs inserted in to their database by other means.
I never found out how they did it, but judging by this topic, it's still possible.
Well, it's indeed possible to add fake transactions in wallets/database but not the blockchain itself.
The link on this topic however leads to an "
Oops page" and OP mentioned that it was a result of a command (
that I guess he'd executed).