There's one major inaccuracy in this story.In Massachusetts Joe Kennedy (D-Neck Brace) is even more of a Maoist than is Markey.
Pretty much everybody votes the way the party wants them to nowadays. Markey does vote solidly for what the party wants, and Kennedy may have a vote a year that strays from the party line. It's not really significant, though. If Kennedy replaces Markey, he will vote as he has been voting and as Markey has been voting. That's what they do.
Markey bought into AOC's Green New Deal in a big public way and Kennedy hasn't, and that's probably what the author is referring to. He may be wrong in viewing Markey versus Kennedy as a left versus center matter, but I don't think the evidence bears out that Kennedy is further left than Markey. If Kennedy wins, it would be because of his family name, not because of issues or convictions. It's more or less the same contents in different packaging.