whoever said you don't block kicks with your forearms in muay thai has never trained muay thai. In muay thai you designate you legs to blocking every roundhouse kick below you head. calf strikes, low kicks and body kicks can all be blocked with the K block.
However, roundhouses to the head, while best to be avoided, are supposed to be blocked with a double forearm block, meaning pull a front block up then rotate it to whatever side the roundhouse is coming in from. it is bad muay thai technique to block a head kick with one forearm, but faster and gets done all the time. But yes, it is accepted technique to block a head kick with a forearm, but the preferred method of a kick block is a double forearm block, thus spacing the impact out over two arms rather than absorb it with a single forearm bone.