I deleted another comment, but this is a far simpler analogy. As a nightclub owner it is prudent to hire a bouncer. Why? Not because the bouncers are making trouble outside your establishment (as a racket would do) but rather as a form of insurance against future miscreants.

Hiring a company like HackerOne (we've hired a competitor) is simply a very wise idea - that is all their email is pointing out. You are currently running a nightclub with no bouncer, you are currently vulnerable (all software is). Get some help by contracting or hiring.