My assumption was that the 6% guild bonus was additive, for a 26% chance of tribute per kingdom (of which I believe there are 27 at the moment). In that case, it’s 1 in 21,231. Still quite amazing!
But if it really is 21.2%, you just had a 1 in 363,812 event!
Edit: My assumption was wrong. As discussed here Can you please verify that Kingdom Tribute chance (w/Guild bonus) is working as intended?, it looks like the tribute chance 21.2% at best, and certainly not 26%.