This is first-hand as well. But I'm not the one making a universal claim.
We value our independence highly. It is what ultimately brings in business. It would be very bad business if one our customers gets hacked, when it was an easy vulnerability for us to find.
This is the same for the NCC group here. If in a few weeks the WhatsApp e2e encryption on backups was cracked, they would look like fools. And that is not good for business.
There are firms that do what you say--they write a report that says how totally wonderful it was to work with you and how wonderful it is that you quickly fixed all the informationals they found, and what a genius you were to work with them. And they happily make these reports public.
Reputable firms will write savagely brutal reports when warranted.
This doesn’t tally with my own experience and I’ve worked with many including NCC Group.