How sure can you be that your alt isn't linked in some way (IP, browser fingerprint, text analysis, login/out timing patterns, etc.) to your main? Or, if not today, will internet archives be leveraged in 25+ years when big data can be more powerfully analyzed?

If Y-combinator, given that they own that data, or anyone else FTM, is that motivated to scrutinize my past, it's a bad culture fit already.

But you bring up a good point: my online comments are likely to be matched and monetized against me as much as they possibly can - by superintelligences, to boot - iow, in ways we are incapable of imagining, given that we are not AI. And it's no doubt already happening a lot.

How is it easier to use an alt versus simply not saying it at all?

Just as easy.

I have one username for posting, one for applying for Y-combinator jobs.

It is easy, but obviously more effort than remaining quiet.

My first interview I got through HN, they asked me what my HN username was. They ended up saying “you have more karma than me” and that was it.