I did not see this case but somehow doubt that it was returned only for this - but obviously could be wrong, I didn't get that letter.
Like this news article that implied one was returned because parents didn't have email address. The article did NOT say exactly this, but implied it.
The one thing we know for sure: not signing where needed is a guaranteed return. It's a legal document, must be signed. (Confusion I think is because 'electronic signatures' applies as far as I know only to documents _submitted_ electronically - but the forms are not clear about this)