I am an endorser, yes. If I didn't like the NYXL strings I wouldn't have bothered to post. Yes, D'Addarrio 'allows' endorsers to be honest about what we like.
Honestly, as a professional musician my reputation and integrity are all I have to "trade". Almost any decent musician can play as well or better than me. What I've done is cultivate relationships of trust with my employers and colleagues. That particularly extends to my expanding career as a repairman & bass setup specialist in Nashville. Players actually come over just to pick my brain and talk about gear (usually the conversation goes to "I don't think you need that.").
I stay in contact with my D'Addario rep and company management every year. No one ever asks to "push" a product or make disingenuous claims. They really do seek out artists and musicians that already enjoy their product and develop 2-way communication.
I'm an open book. I abandoned anonymous Internet personas years ago. I'm willing to backup any claim or experience I communicate online with who I am on planet Earth.
I understand your line of questioning. Being a company 'shill' isn't how this works though.