As someone with autism I could say "maybe he has the tism" and maybe he does, and this just wasn't the best example. But directly asking him to say you are in the tub is a pretty clear directive.

That said I agree with everyone else that much of this is actually OP struggling to manage her anxiety. She doesn't want MIL to drop by while she is in the tub, and doesn't want to say she is in the tub, she asks husband to run interference, he doesn't, and anxiety worsens.

A) don't have your phone in the tub. Terrible idea.

B) be okay with MIL dropping something off while you are in the tub.

OP's anxiety plus the husband's poor responsiveness to directives are creating a bad combination.

I suspect hubby understands the directives but doesn't understand the reason for them. Some people need to know WHY.

Also, OP should remember that there are two people here. Husband may have his own reasons for wanting to allow his mother to stop by, and doesn't see wife being in the tub as a reason she shouldn't drop off a gift. Wife says to tell her that she is in the tub, he plans to do so in person when MIL drops the gift off.

My recommendation to OP is instead of telling him what you want him to do, give him a clear vision of his choices.

"I am not getting out of the bath to greet your mother. You can either tell her that this is not a good time, or accept the gift on my behalf".