We just did this tour in October, saying at Boardwalk. Prior to the tour day I had heard about varying degrees of success trying to get in via International Gateway (I think the tour email even said we could use IG, but that info is only as valid as the CM manning the entrance, and apparently not all of them are aware of this tour or how it’s supposed to work).
Long story short, we had elderly parents with us and didn’t want to walk all the way down to IG to risk being told we had to go back, so we just sucked it up and got an Uber to the front entrance of Epcot.
Interesting related side-note... technically the tour ends in Magic Kingdom and then they drop you off back at Epcot, however a significant chunk of our tour group requested to stay at MK instead (they were dropped off by the bus depot in front; of course you would need valid admission to get back into the park, but my parents actually used this option to take the MK bus directly back to the BW hotel so they didn’t need another uber from the front of Epcot). I’m not saying this is the norm, but it seems to be something they might be willing to do; YMMV.