Let's face it, "tradition" is a fluid term. "The way we've been doing it for years" can fairly be called "tradition", for the particular group concerned, at least. That, of course, may be very different from a "tradition" passed down within a culture from time immemorial (supposedly). Instrumental pub sessions, particularly those of the "Irish" variety, have been going on for long enough that it is probably fair to apply the terms "tradition" and "traditional" to them, with the understanding that that does not mean they have "always" occurred.

Of course, I'm not telling anyone anything they don't already know - I just felt the urge to pontificate.