It may not be. In fact, I'd suggest discussing the idea of validity in relation to opinions is probably fraught with peril itself. That said, any opinion which is more close to the unbridled truth is generally more structurally sound (and maybe that's a good hallmark for "validity") than opinions which are more sensationalized.

"Grey opinions" are essentially the opposite of sensationalized. I'm not the original commenter, but if I read them correctly, I believe they mean that the least sensational and most deliberate opinions are more likely to fully account for the truth of the matter (and hence be "more" valid) than those opinions which sensationalize.

I would assume the author of that comment was using "grey" to describe an opinion that is more nuanced and carefully considered. A black/white opinion would be hyperbolic or simply expressed to provoke reaction.

So if that's what a grey opinion is... then yeah. That's more valid. Especially if you value conversations aimed at exploring some reality rather than performative shouting matches.

because the person you're replying to agrees with that opinion at the exclusion of all others