For example, could you today run a ruby based SSG from 10 years ago? Let's pretend there is a bit of custom code and time invested in it.

I guess you could install a bunch of old versions on Ruby, gems, etc. on your machine too, but docker, or a VM might be easier.

Or you could go through, remember how all the code works and upgrade it to the latest versions of everything to run it. Assuming the SSG du jour 2013 is still maintained :-).

I tried to resurrect a 2 year old NodeJS project and gave up trying to upgrade it to the latest versions, NodeJS people really love their major version changes, and breaking back compat for no real reason.

You don't need docker for that. Also if you leave a WordPress for years with auto-updates, the plugins will often be abandoned causing your site to be vulnerable.Not to mention any breaking changes that can be introduced.

A SSG generated site will be the same in 10 years as its today, even if you might need to fix some things with your project to make further updates.

You basically just said wordpress will bitrot but SSG tools are immune.

He appears to me to have said they bitrot in different ways. Which way is preferable is a matter of priorities.

Reading it again I think his point is that the generated HTML is good enough to spring further work even if you lose the original tool. I can agree with that.

With wordpress you could export it to HTML as a backup but it is an extra step.