Though more recent evidence does seem to show this is incorrect, this was the best guess that experts had for the original meaning of
Brendan when they were still thinking it was derived from ancient Irish Gaelic.
More recent experts now believe it is an importation into
Ireland of a Welsh word meaning "prince."
This is a good example of how more recent research that finds new evidence can change the accepted origin of a name. So Nameberry is correct that "older sources" give "stinking hair" as the original meaning of
Brendan. It's just that those older sources have now been overturned by new evidence.