Anyone who walks around calling themselves a master whether self taught or not, isn't a master.
We overblow the master thing in martial arts. Master does not mean perfection, nor does it mean ultimate fighter. To 'master' a skill means that you have become proficient in it. The term is used all the time outside of MA and nobody gets all lofty or philosophical about it. Being addressed as a master is, in some organizations, part of earning a specific rank. So long as people do not let it go to their heads, it isn't really an issue.
The same holds true for any other title one might be addressed with. I've seen managers of small retail stores get caught up in their title of 'manager.' You'd think that they were the president of the company. Nothing wrong with the title or being addressed as such. Nobody rolls their eyes when your doctor instroduces herself as 'Doctor Smith.' She earned the title.
Now, guys who are self taught and look like the gent in the videos I saw on that other site calling themselves master are just plain silly. But plenty of people of dubious skill, background, and education call themselves doctor as well.
There in my opinion is no such thing as a master just people who are better than you or better than some. The old saying rings true "no matter how good you are there is always someone better" So no one is a master.
A master carpenter, mechanic, electrician, or whatever is a master by virtue of what they have learned and the quality of work that they do, not based on being better than everyone else. They still learn new things year in and year out as they spend more time in their trade and as the trade changes with the advance of time.
Likewise, a master in martial arts is generally a master by virtue of knowledge, time in the art, and presumably quality of their practice. This in no way implies that they have learned everything or are better than everyone.
Now, I do agree with you that runnng around calling yourself master frivolously, such as on Youtube, is pointless, silly, and is asking for negative criticism, as calling yourself master on a youtube vid will insure that virtually every deviation or mistake that you make in the video, no matter how minute or what the reason, will be scrutinized and pointed out.
Daniel