I largely agree with a lot of the article but think it's a bit more nuanced than that. We've seen practical examples of painting the other side as extreme not working. For example back in 2020, republicans tried to paint Raphael Warnock, a pretty middle of the road Democrat, as a "radical leftist" and it never stuck. On the other hand, his opponent was actually a crazy Trumpist and lost handily as a result. On the opposite side of the spectrum, Glenn Youngkin was painted as a crazy right winger in 2021 when he was more of an old school "normal" Republican. It didn't work and he won a race where the left was largely perceived as extreme on COVID and education issues.
I think the takeaway is more that accusations of the other side being extreme don't really land because it has become the default setting. But it's not a meaningless term and independent/swing voters do vote against candidates and policies that they consider extreme. Republicans are largely seen as extreme on abortion policies right now and it has continued to cost them winnable elections. It's just that swing voters don't need to hear that message from partisan operators because they won't always buy it (and a lot of those messages are really only going to be heard by extremely partisan voters anyway).
I also think painting Trump as an extremist doesn't really work, particularly for many of the reasons outlined in that article. Trump irreverently voices a lot of things that people actually agree with. His downfall is when he is painted as incompetent, incapable of delivering on his promises, or just plain dishonest about doing what he says he's going to do (the border wall sometimes seems to be the most effective sticking point against him).