Your question relies on a false assumption: "while all Stargates on Earth were sealed away."
Though the Antarctic Gate was under ice in the 20th century, there was some indication that it was usable before that.
In fact, Daniel's line of reasoning to find the second Stargate (where O'Neill and Carter were lost) relied on it:
JACKSON: How many Earth-based cultures have we encountered from other worlds from periods both before and after we think the Stargate was buried?
HAMMOND: Several, I think.
JACKSON: Right, and we've probably only scratched the surface. Now, so far we've tried to account for these discrepancies with various theories of parallel or accelerated...
HAMMOND: What's your point, Doctor?
JACKSON: What if there's a second Stargate
JACKSON: It would have to be in a remote location otherwise it would have been discovered by now. It could have even been buried until recently otherwise the Goa'uld would have continued to use it.
Which suggests that it was buried until recently, but not necessarily that it was buried ALL the time until recently.
In the episode "Demons", there is this conversation:
CARTER : Well, this is the first sign of Christianity we’ve encountered, Sir, in hundreds of missions.
DANIEL : Which means they probably had to have been taken from somewhere in medieval Europe through the Antarctic gate.
This is no longer pure supposition, this is after they know about the existence of the second gate and presumably looked into its history. That Daniel comes up with this theory suggests that, based on the evidence he knows, he doesn't believe the gate was sealed then.
I can't seem to find a source, but I also have a fleeting memory of a scene where one post-Egyptian culture had a history where their god led them through a long journey through a place that got colder and colder, with the suggestion that this was their journey through the Antarctic Gate.
Finally, although there's no explicit evidence, we do know that Goa'uld sometimes do take Stargates WITH them on ships. A Goa'uld that wanted to set up its own empire could have taken a very small, fast ship, large enough only for a single Stargate, to a highly populated planet, then, use the Stargate on that planet to ship slaves to a new world. So any other space-faring race or individual could transplant people in this manner rather than taking a long space journey.
So, with respect to the Arthurian legends, there are several possibilities, including that the legends originated on Earth and some of the population was merely moved off-world, or that Arthur himself recreated the legends multiple times in multiple places, but both might well have used the second Stargate, which at that time did not seem to be sealed.