I feel a kind of passive aggressive vibe from you OP! Why does it annoy you that mothers often mention that they cannot get to pee in peace?
Is it because it implies that staying home looking after toddlers is not always a bed of roses, and that SAH mums sometimes actually do have stuff to deal with that is not enjoyable?
I know people who are determined to maintain their view that SAH mums are useless and lazy and do as they please all day long without a care in the world. Such people will not admit that a complaint like not being able to use the bathroom in peace has any validity, because then they would have to admit that their view of SAH mums might actually be wrong!
Being home all day with babies can be mind-numbingly boring and filled with little gripes like this, and also wonderful and delightful at times. Just as going out to work can be tedious and thankless, and at other times rewarding.
And yes, of course, anyone can lock the door and be physically alone whilst using the loo. But you cannot necessarily be MENTALLY alone.
For the first, say, 40 seconds, a small person may bang relentlessly on the door. Then all goes quiet. Then you hear an unusual thud. Then you call "are you ok out there?". Then there is silence. Then you reach up from your seated position to unlock the door and peer out. And either some little trickster comes barging in and you are no longer alone, or there is no sign of anyone and you finish hastily and
head off to assess the damage!!!!
In short, OP, yes, YABU. When in sole charge of one or more little ones, YOU ARE NEVER ALONE......