I would be really surprised if that was the problem when the One S was also different than the original Xbox One. For me it was either them wanting to have a way better console than the Pro, and willing to wait for that to become possible, or being taken by surprise and deciding that having a strong answer, even if late, would be better than a worse console that would be out closer to the PS4 Pro.
The problem that Xbox have is that no matter what it will get difficult for them. From a marketing standpoint their offer is a premium product, and a entry level one. Having another one would mean changing a lot of their messaging and would be fascinating to see. If there is a Series S, X and a U for ultimate, how to market them all well and be consistent when those consoles are fated to hinder each other?
At the same time, yes if there is a PS5 Pro but no Xbox equivalent their S/X strategy will become a bigger problem. I think that is why we have people here like
@HeisenbergFX4 that say that we will have a early next gen from them. They will IMHO compete on price in the meantime, and that will bring another kind of problem to them as I think that the Switch 2 will really hurt the Series S market potential if Nintendo is smart.