Although Roy had a vague guess about the current situation, he wasn't sure if his guess was right. Moreover, this guess was only a general direction, and he didn't know the details at all.
But in any case, this unexpected situation gave him a huge leap forward in his plan to join the Burning Legion.
It was true that the Burning Legion was a demon army, but this didn't mean that every demon who came to ask to join could be placed in an important position in the Legion, even if they were as powerful as Roy, a demon king. As for why? It was because the eredar demons were the managers personally appointed by Sargeras!
In other words, in the Burning Legion, only the eredar demons were the leaders, and they were the main faction. Under normal circumstances, even if Roy joined the Legion, with his status as an Abyss demon, he would at most be the demon in charge of the Abyss lords. It wouldn't be easy to enter the true core of authority.