And did they hold a couple more years? Or for some reason they sold at bottom after averaging down?

Most dot-com stocks went to 0. There's a famous "99 percent club" of stocks that became wallpaper.

but if you followed the advice of modern portfolio theory, you would've purchased a diversified set of stocks (cap weighted) during the dotcom bubble, including companies such as google, amazon, and apple.

I think even if 99% of the purchases went to zero, you'd probably still be ahead if you're diversified properly.

Google went public in 2004.