Different contexts, that’s where it comes from. Just because the comment they responded to said “Mordekaiser is the only champion you can safely attach to” doesn’t imply that only Yuumi and Mordekaiser are around. It just means Yuumi feels like she can only attach to Mordekaiser safely... whether he is right or wrong or if it’s his fault for being in that position is a different problem of course.
In your case, if Yuumi and Mordekaiser are the only ones around when the enemy collapses on them, it would be nice. I don’t think it would be a good change overall because in the mid to late stages of the game, Yuumi shouldn’t be in a position with just one ally. That’s not really playing to her strengths and using her effectively.
Anyways, during a team fight, if Yuumi is attached to him when he ults, a 5v5 could turn into a temporary 3v4 in favor of the opposing team while Yuumi literally doesn’t contribute in any way through this change.
In a team fight scenario, one could argue “But I don’t want to detach from Mordekaiser because I might die!” I feel like if you’re going to implement this change, however, you might as well take the potential risk of going for a reattach because if mordekaiser isn’t popping his target immediately, you’re probably going to lose the fight anyways because you’re figuratively AFK for so long.