I don't think the nobles need to know each other, or even of each other, for the ability to make sense.
Sure, and those nobles shows up with men-at-arms, servants, goods, etc. It was an entire train of people that commoners could not match. Not some lone guy with a few scruffy friends showing up and making a claim.
Here is the text of the ability:To be clear here, I don't think you are wrong on this. I could honestly see arguments for both positions. I'm just fine with the arrangement that the GM is the one who gets to decide that
Thanks to your noble birth, people are inclined to think the best of you. You are welcome in high society, and people assume you have the right to be wherever you are. The common folk make every effort to accommodate you and avoid your displeasure, and other people of high birth treat you as a member of the same social sphere. You can secure an audience with a local noble if you need to.
And here is some of its accompanying descriptive text:
You understand wealth, power, and privilege. . . . Nobles are born and raised to a very different lifestyle than most people ever experience, and their personalities reflect that upbringing.
No reference is made to being known. Or to having an entourage. It refers to birth, to understanding, to personality. These all imply comportment. Much as the CHA stat - a long-established part of D&D PC build - does.
I don't see how the idea of a "GM ruling" even comes up! The rules text seems clear to me.
Here's more of the background text:even if the GM has a weaker reason than the player, my general feeling is "its the GMs setting and he is the one running the campaign, so let him decide".
Work with your DM to come up with an appropriate title and determine how much authority that title carries. . . Not only do you need to determine your noble title, but you should also work with the DM to describe your family and their influence on you.
It's clear that the player is being granted some authority here. Just as one might expect from a player-side rule for including background as part of PC-building.
The view that, despite all this clear text, the GM gets to gate the background feature behind their own view of how the fiction "should" work does not seem to be in contrast with "Mothery may I". It seems to be defending it for this particular context.