There's a line from a show I like: "I figure there are always going to be reasons not to do something. If you really want to do it, then you have to do it."

It sound simplistic but honestly, this happens in life. I feel for the author as someone in the same situation (left academia for industry). But at some point it is what it is. You can't change reality. You can't reverse time. So in every moment you just have to decide, "Given the world is this way, what do I want to do?" The world is going to continue on day after day and is not going to stop or fundamentally alter itself to align with your preferences and well-being. All you get is the choice.

He left in 2013, most postdocs positions have a 5 year cutoff for eligibility. It's been 7 years since he got the degree so a lot of doors have already closed for postdoc opportunities. Professors will find reasons to not hire him such as being out of the game for too long and thus he's not caught up in the literature or whatever. He could try to go in as a data scientist. It is true though that usually that job goes to students because they're cheap and capable, they just take longer. So yeah it's kind of hard to compete against cheap, passionate labor that students provide.

I don't even think it's most personal--it's just a lot harder to pay someone who isn't eligible for most fellowships.

I have no idea how that blatant age discrimination is legal but....

He becomes a student again and takes on a slightly different postdoc if he wants to go back to that level.

Fairly few places will let you do (or fund you for) a second PhD.