And your current definition of "restraint" fits within the articles use of the term. Putin has shown restraint because of a range of possible factors (fear of NATO response, Ukraine air defenses, poor supplies, desire to preserve rail for his own use, etc.).
The articles use of the term restraint doesn't mean the authors feel Putin is altruistic. That should be clear to anyone who read the first sentence of the article, "Russia’s war against Ukraine has leveled cities, killed tens of thousands of people and forced millions of others from their homes."
No one is saying restraint is synonymous with rationality. However, restraint is clearly a characteristic of a rationale actor. That is the main thrust of the article - which I suppose is what you may take issue with.
The article is not solely focused on restraint from using nukes. It is about restraint in terms of cyber attacks, attacks on Ukrainian railways and bridges and other infrastructure, limiting the current offensive to a slow grind in the east, etc.
Trying to understand Russia's decision to pursue more limited aims is clearly helpful. It is clearly helpful to understand why he is constrained - why he has shown restraint in certain areas. If folks are only comfortable seeing Putin as a non-rationale actor with no ability to restrain actions, then that isn't helpful IMO.