Not the poster of the question,

But I think they're referencing the litany of transpilers and repackagers which exist for js. So you can add new features and then still have it run on really old systems like internet explorer 9 if you need to.

This has problems obviously and in my opinion for python it would be preferable.

My reasoning being that if you need your code to work on an older system being able to write and use current syntax is preferable to not, and the hard bifurcation that python did with 2 to 3 and now potentially with 3 to nogil seems to me just to break apart the ecosystem more.