Well in comparison to his equals in boxing and MMA he is. Do I think he's broke? no. Do I think he could probably be paid more? yes. Is it possible he's doing these seminars for the experience and not cause he needs the money? of course but if it was just about the experience he would be doing it for free.
But boxers and mixed martial artists do do seminars....top tier actors do seminars. top tier businessmen do seminars. top tier athletes from larger spectator sports all do seminars.
There is no reason why he can't teach something that he loves while getting paid. They're not binary. People do things they love to get paid all the time. There is literally no downside to him getting paid. Getting paid to teach is not desperation.
Rodtang making less money than his MMA and boxing equivalents doesn't seem relevant for the same reason why an MMA making less money than a boxer doesn't seem relevant. Muay Thai is a domesticated spectator sport that is based in a developing country where it is niche. Yes MT fighters make less money, that isn't why he was doing a seminar in another country.
Rodtang makes 300,000 dollars per fight, and due to his fighting style usually gets another 100,000 or whatever as just bleed money. He is booked to fight 3 times a year. He is a millionaire in a country where the average person makes 700-800 dollars a year (and that's including people from Bangkok which has actual industry).
Rodtang and his MMA equivalents are WAY closer to each other financially than their boxing equivalents. UFC fighters are only booked twice to fight a year and usually make 600-700k per fight officially. Many of the best fighters are in Bellator and have purses similar to Rodtang.
Rodtang lives in a province in Thailand while most UFC fighters live in the United States even if they are from somewhere else so Rodtang saves more than most MMA fighters do by a lot. (the UFC has more sponsorship friendly contracts which is saying something and of course PPV points, so yes the top UFC guys have more money than Rodtang but in the grand scheme of things it isn't even by a lot which tells you how underpaid UFC fighters are).
You would actually have a point for many top tier Muay Thai fighters but Rodtang is a not a good example lol. If you think Rodtang has to go to Canada because he needs money when he lives in a place so cheap where he couldn't even spend all his money if he wanted to then it feels like you're projecting.