Yes, no doubt, that IS boosting.
HOWEVER, it's a permitted form of boosting.
Because to outlaw it, it requires Riot to ultimately acknowledge the notion of smurfing to be basically illegal itself, or that they have to impose it so smurfs can not duo with anyone. Which neither of the 2 is going to happen, and thus it's allowed as if floating in a grey limbo.
It's still despicable imo, not just this boosting but the notion of smurfing as a whole. Smurfs are literally what makes elo hell even a thing and screw up low elo matchmaking in more ways than 1; not to mention how abominable the mere notion of it is, it's like a professional boxer going into a some casual boxing club uninvited and piss on everyone there... what's the point?