I don't think he does at all. Teams need three quality rotation guards. Smart didn't start playing point until college, and he still has a lot of development left in that part of his game. I saw some pundits calling Smart undersized at the 2, but he's capably guarded small forwards this year, so I don't give that much stock.
The analyses done by Celticsblog and others leading into the trade deadline indicated how some of the Celtics biggest weaknesses are Thomas' strengths. Smart ahousk still get his 30 minutes per game most nights -- no development will be stunted.
From a medium-term point-of-view, winning this year is okay. The Celtics are clearly too good to finish in the bottom 5 barring trades that ripped apart the team, and there was little point in that. If they win a few more games, so what? And if they make the playoffs because they've continued to play better, fine. The goal is to win eventually. What's wrong with now?
From a longer-term perspective, the Celtics now have their main backcourt rotstion locked up through June of 2018 for between $18-$19 million per season. That means they can spend other resources (draft picks and cap space) improving their frontcourt, unless and until an even better backcourt upgrade comes along. It takes up a little more than 1/4 of this year's cap, and close to 1/5 of the following year's cap, to cover 3 of their top 9 rotation pieces. From a team-building perspective, this is very good.
I frnakly think this writer was being contrarian for the sake of clicks. The Thomas move made between a good deal of sense and a huge amount of sense. The only thing I'll concede is that his reputation in the locker room isn't perfect. That is a definite risk. Still, Bradley played with him on the same youth team, so hopefully that decade-long relationship makes IT feel more at ease.