There are other sources but they all say the same thing. At this point, I'm leaning more towards the story being true rather than the existence of some global conspiracy orchestrated by invisible superpowers trying to rewrite history.
This is from the official website of Singpatong Sitnumnoi gym in Thailand:
When Somrak reached 18 years old, he had more than 200 bouts under his name. But despite his remarkable ability, Lumpini championship win evaded him. Even at the smaller Rajadamnern Stadium, Somrak never became a champion. Somrak was considered a prohibitive favorite which caused the big gamblers to lose interest in him. The promoters did not want to risk their prospects against him. Somrak suddenly found himself out of job and he could not support himself.
http://www.singpatong-sitnumnoi.com/art-of-muay-thai/
I also wanna add that the second link I posted is an interview of Somrak himself, who is certainly the most reliable source on this matter. If somebody was to investigate Somrak's career, who would be the first person they'd talk to? Somrak himself, right? Well, Somrak has spoken and this is what he said. I don't think it's fair to qualify his words as mere "couple of lines thrown out there on the internet".
It's also important to note that each link contains information that the others don't provide, which shows they are not just plagiarizing one another.
