If you're only playing with your alts on the team ladders that is allowed. It could be different through if u play in a normal team with one of your accoutns, lose a game there and rejoin the ladder with your alts and that account. Than u would have unexpired games on that account, so according to the sentences above this seems not to be allowed.
That would be a bad rule. Now I play with alts on 2v2 ladder. What if I wanted to start a ladder run with a friend? I have to wait 5 months? And then, I play with him, then he loses interest. I need to wait another 5 months before entering the ladder with a different teammate? That makes no sense.