I wouldn't believe the Thai numbers. At the start of the pandemic they were one of the first infected countries and their number of cases stayed at 80 cases for ages while the numbers for countries like Iran and Italy went through the roof.
I'm sorry but for a country this close to China which has so many worldwide tourists and which didn't close its borders to Chinese tourists or other tourists, I don't believe they've only had 3k cases and 58 dead, or that they only have 2 to 4 cases a day now. I don't think they're really able to keep a good tracking of infected and dead to be honest.
I don't believe the official numbers either, of course they are higher than the official numbers. But if you look at the excessive death rate in Thailand it's still very low.
I would consider the limited clinch to be "dumb as fuck" if it was the only, or one of the few precautive measures implemented, or if it would have very bad consequences for the sport. Although it seems very likely it will not have any impact on the transmission of the virus, at least there is really more to be gained than to be lost from such a rule.
First of all, the chance that either of the fighters has the virus at all is probably low. Second, we don't know the frequency of transmission in Muay Thai fights. If one of the fighters has the virus, will it transmit 99 % of the times? 50 %? 10%? At this point we simply don't know.
Two things to consider: 1) the time spent close to someone that's infected, and 2) the size of the viral load that's inhaled by the other person.
It seems reasonable to assume that time spent close to someone is positively correlated to the size of the viral load. Furthermore, the size of the viral load transmitted may affect the immune system's capacity to handle the virus.
Some people here pointed out that the rule is dumb because you will punch someone on the mouth either way. However, does this mean that regardless of limited clinch rules or not, the viral load will be exactly the same? To me it seems perfectly logical that clinching less will tend to transmit a smaller viral load in general. Maybe it doesn't but given that we don't really know, it's worth a try at least. People said masks were inneffective because they don't block the virus, but now most researchers seem to believe even masks are not effective at individual level, they will reduce transmissiong reatly at population level.