Actually, that’s not an unwritten rule so much as the fact that, if you make a direct comparative claim for your product versus a specific other product (e.g., “Tide is better than All”), you have to have some very specific (and fairly expensive) market research to support that claim, or it won’t stand up to legal scrutiny.
If Tide, for example, were to start airing “Tide is better than All” ads, the makers of All would file a complaint with the FTC, and the FTC would inform the makers of Tide that they would need to either (a) provide the FTC with the documentation of the research that supports that claim, or (b) cease running ads with that claim.
If, on the other hand, you use “Brand X” or “the leading brand” in your ad, you aren’t mentioning All by name, and the makers of All are much less likely to be successful in trying to force you to stop the ads (since you’re not referring to their brand).
I’ve spent my career working in market research and advertising, and I have seen at least a half-dozen situations in which a marketer was interested in making a direct comparative claim like this in advertising. There’s no specific rule from the FTC that says “you need this specific piece of research to support such a claim,” so much as a history of cases in which claims have been upheld, and the specifics of what sort of research was considered by the FTC to be sufficient.
Anyway, in every single one of these situations I’ve been involved in, when the marketers were shown, “OK, here’s what you’re likely going to need to do as far as research goes, this is roughly how much it’s going to cost, and this is how big the difference in performance between your brand and the other brand is going to need to be to support such a claim,” their interest has quickly died away. In my 30-year career, I’ve never actually seen this sort of research conducted.