Well... that's the official story anyway. The truth is more complicated as it often is in these matters. He has engaged in buying
"non-productive assets" when he bought silver. I suspect if he knew a way to value BTC, and thought he had an edge, he'd buy it. I don't entirely agree with people who want to proclaim that crypto has _no_ value, though I'll be darned if I could say what it is exactly.
He (or his team) also
bought Snowflake, which is an emerging cloud company at it's IPO. In that case I'd guess the edge was being able to purchase before the IPO. There also a lot of speculation this was one of his team, though he supposedly okays all deals.
The other criteria you're missing, which forces him into mature companies is that it's just not possible for Berkshire to do well buying anything buy small companies. All stock prices are set on the margin, so any moves to acquire a piece large enough to make it worth Buffet's time is going to skew the price so heavily as to destroy any margin of safety. This is unfortunate, since it's likely that as you said a lot of promising companies are likely smaller cap companies potentially in emerging fields.
Also define "tech" or "technology"
I'd argue that some of the current "tech" companies really aren't or weren't. Amazon is a good example, they're got issues, mostly with size, but doing Sears Roebuck and company over the web is hardly innovative. Facebook is similar to Amazon, they're a BBS or a message board on the internet. They've also got some issues with scale, but otherwise it's not a really amazing company, and honestly their product has often been lamblasted in tech circles for it's poor implementation.
Apple is in a similar boat, having always taken what others did, and just add some style to it. They're the Gucci of "tech" companies. And before any of the Apple fans get upset, if Apple is a "tech" company, why isn't Precision Castparts, or BNSF, or GEICO? They're all "innovating" at the same rate as Apple, maybe not as flashy, but still.
OTOH, there's IBM, which has been around for forever, as well as Oracle and Buffet could have probably bought either one, but didn't.