Spoiler ShowNomura: Yes. Long ago, there existed Keyblade Masters called the "Foretellers", who traveled through time to be brought together again. Xigbar, as well, is actually another Foreteller named Luxu, who continually changed form through the ages.
So I've got a theory on how the time travel works. Since Darkness from KHUX mentioned there's a different way to travel through time, I'll give it a shot.
To put this out of the way first, Xehanort's time travel works by having a version of himself existing at a certain point in time, where he wishes to travel.
However, the foretellers cannot do this same method for KH3's epilogue, since they don't live past the era our current cast exists in. But Darkness from KHUX presented another method though.
What if, this method, draws on the power Sora specializes in: Connections?
Ira in both the English and Japanese version of the epilogue, specifically mentioned that Luxu 'called' them at that point in time. So what if by utilizing the Connections of the Heart, the foretellers are able reel themselves in the point of time Luxu called them.
Actually, it works almost like a summoning to sum it up. Like Luxu summons the foretellers from a specific point in time to the time he wants, but what requires is that their hearts must have a connection with the 'summoner' before hand.
The way how I think this is how it works is base on Maleficent's come back in KH2. Nomura already confirmed in the KH3 Ultimania that the Maleficent we see in KHUX is post-KH1 defeat, pre-KH2 revival. And the way Maleficent was revived in KH2 was because of the three fairies connecting with Maleficent by accident, despite their hardest not to think about her. Maleficent utilized that connection to bring herself 'back to life', but as we know it now, time-travelled back to the future at that moment.
Thus the method of this time-travel requires a connection towards the period of time the travellers intend to go to.
This then sort of begs the question, how did the Union Leaders time travel? I do believe it's the same method as this, provided if it's true anyway. Someone called them from the BBS era or the present time, and thus were summoned there. Albeit a slight variance in the point in time they landed in, as well as the location, all due to being unskilled like Luxu mentioned.
Why do I think it's the same method? It's because of the hint that Brain was the only one who succeeded in jumping to another worldline, without time travel. Of course provided again, if my theory is correct.
Someone who knows of the Union Leaders summoned them in the BBS era. But since Brain at this rate is already hinted to be a fake, whoever knows of the Union Leaders in the BBS era timeframe either doesn't know Brain, knows specifically he isn't a Union Leader, or only knows of the original chosen Union Leaders, thus he never was called upon in the BBS time.
So this makes me think that the one who summoned the Union Leaders in the BBS era, has to be Ava herself. Which most likely means that she's body hopping like how Luxu is.
Although, it could just mean the Union Leaders accidentally time-travelled though without using the same method as the foretellers did. Or they could've used it, but used the wrong conditions or did it the wrong way like what Sora did with the Power of Waking.