There is no way to excuse that accusation based on just where the guy lives from Elon. Not if he were an ordinary guy and especially not if he played an instrumental role in the actual rescue. It seems to meet the standards of defamation even under strict US interpretation. Elon owes the guy a full apology at least and possibly monetary compesnation as well. On top of that, it's also insulting to the people and country of Thailand. Like Elon said just a few days ago : "I probably should say nothing more often". He let an excellent opportunity to say nothing slip away.
"In a series of tweets responding to a journalist Musk said: 'Never saw this British expat guy (sus) who lives in Thailand at any point when we were in the caves. Only people in sight were the Thai navy/army guys, who were great. Thai navy seals escorted us in — total opposite of wanting us to leave.
'Water level was actually very low & still (not flowing) — you could literally have swum to Cave 5 with no gear, which is obv how the kids got in. If not true, then I challenge this dude to show final rescue video. Huge credit to pump & generator team. Unsung heroes here.
'You know what, don’t bother showing the video. We will make one of the mini-sub/pod going all the way to Cave 5 no problemo.' "
I see no defamation. There's a non-specific opinion, and a list of observations. If you want to call him out, trying tweeting and asking why he wrote sus.
If the guy _knew_ that rigidity of the design wouldn't work, then he could have tried to communicate the information directly. Just a length limit would do. One of the benefits about doing it in public is that people know what they're attempting. With direct contact (even via twitter, it couldn't have been missed), they would then know to focus on a flexible solution.