The idea behind the phrase is that when a ton of speculation money is flooding in to a market, you should make money by proving services to the speculators, not by speculating yourself.

Creating mining hardware is actually more speculative than an exchange, because you're picking winners and losers based on what your hardware can accelerate.

Selling pickaxes to miners has been used to describe platforms like CDNs or game engines like Unity. The fact that there is something called a miner in this context is a red herring.

A metaphorical one, I mean.