The sms verification means people will need multiple cell phones if they want to try to smurf and also be able to get clash rewards on their main, so it seems like a pretty strong incentive to avoid playing clash on a smurf account. They did say that playing on smurfs isn't ok anyway, and those found out will lose their rewards.
I wouldn't put the limitation on inactivity though. Theres probably plenty of non-smurf players who may have half quit and might be coming back to try out clash, and besides that it's not terribly difficult for people to just get in a few games in the previous week or two. Most smurfs are actually played on at least occasionally anyway, rather than laying dormant for months on end, so I'm not sure the inactive thing you're describing would even achieve the goal of stopping smurfs anyway.