TLR, for it to truly be opt-in would require ACT to also become an opt-in program. (I.E: ACT would have to include a blacklist of player names not to parse in its function)
He mentioned so long as ACT parses everyone in the party, it'd be trivial for someone to create a second website that'd reveal the non-opted in names of people if he ever made FFlogs opt-in since ACT still stores that data on the parse file. He then explained It's even happened when a bunch of Chinese players created a Chinese version of FFlogs, and that exposed every hidden name on FFlogs anyway since ACT still keeps track of that info.
He also humorously noted how in the many years of running WoWlogs, he's never had the request to hide away data, it's only ever been a request from FF14 players.
I'd call it a fair reason to remove someone, so long as they aren't a jerk about it. This game's endgame content relies all around Hard enrages; mathematical equations that are extremely binary if you fail or not. When someone's out to complete Titan for example, I wouldn't fault that person for being hesitant for not wanting to party with someone that has their logs hidden. Bringing someone in who is obfuscated in their performance is a risk, you could get lucky or not. But between the choice of waiting for someone who has the performance AND shows it off vs someone who may or may not have the performance, potentially wasting lots of time if their performance isn't up to snuff, its easy to see why people would choose the former.
I see it as no different than not wanting to play with someone who claims "they've seen enrage, just need a good group." or "This is an alt." In all scenarios you're taking a gamble, and not wanting to take it due to lack of information available to you is not harassment. (Unless they straight up insult you/claim your performance must suck because you're hiding your numbers, etc, then that's harassment.) That being said, if anyone ever mentions parses in-game, they're asking for a report when they could simply say "sorry X, we got a friend coming" as a fib reason to remove a person.