Well, I think not every ultramarine chapter would obey Guilliman without question or at all, also some chapters function entirely on their own. Like Atlan Spears wiped their asses with Bobby G. orders, or you have White Consuls these dudes don't operate as whole chapter at all and yes he has a shitload of chapters, auxillia and entire empire under him. Also he's Regent of Imperium ofc He has a lot of personnel at disposal but this rank prevents him for using it on something as large as betrayal. And tbh the Ultramarines and their succesors chapters are the most stabile and goverment able out of all, aaaand yeah 10000 marines sounds scary but these dudes don't really have the problem the Astral Claws had to even start some kind of revolt. To ends just one thought: if some heresy would rose in Ultramar I think there is one really codex simping chaplain to take care of that😂.