I don't think there's always obvious linearity with learning, especially not these days where you have more options vs. elders copying programs from magazines line by line or however the legend goes.

How responsible was fiddling with vbulletin/phpbb forum plugins in my eventual career, one of the first things I remember doing and for quite a long time?

I have no idea, and it was just one of the various things I did as a noob, none of them with an impact particularly memorable. There were a bunch of nodes in a daisy chain of things I tried out, and over time I could take on bigger and bigger projects with fewer and fewer rails.