Gotta disagree. The most disruptive up and comers seem to get there through word of mouth. I mean look at Figma. I haven't seen a single Figma ad since they began as a company (they probably exist somewhere) but they really rocketed off through word of mouth among the design community. Pretty sure slack was similar in this regard. Both disrupters.

I have no actual evidence of this always being the case but I would imagine given the fact the nature of a disrupter is that they're usually operating on principles of delivering a better product but without the budget to go crazy with advertising, they have to find more grassroots methods of market penetration.