So looking at that video, he starts with 100% tm and then begins his attack. His tm goes to 0, then after his AoE jumps back to almost 100 again, so I think his AoE grants him tm. Then he does his third attack, and after that his tm goes back to 0 and he attacks 3 more times. After that your characters gain some tm and he starts another round of attacks from taking his next full turn. At some point he gains tm during his turn yet again. It's hard to know since they don't show us his kit, but I'm guessing he gains 100% tm when an enemy is defeated or something of that nature.
From looking at where your tm bars are in between his natural turns and the speeds you listed, I'd say his speed is in the 290-300 range.
So with better gear and mods you should be able to overcome this, but I don't think it's bugged unless they come out and tell us he's not supposed to have tm gain abilities.