Dunno if there's supposed to be a Gasha for the Exp rune or not, but it is one of the possible bonus runes from the Loot Gasha.
I'm guessing this is what's supposed to be the case. The 'promo' listing it is probably just a copy-paste error from a previous promo. Unfortunately, the 'weekend' & 'time of year' means finding that out for certain will be near to impossible.