Mustafar wasn't a thing when that line was written. They made the Mustafar scenes to retroactively look like the line is referring to them, perhaps, but certainly not the other way round. So nothing in ANH to prevent a meeting that is equally, or better, described by that line. It wasn't even delivered as dramatically as deserving to be referring to Mustafar. He was chopped into pieces and left for dead, they haven't met since, and all he got is “I was but the learner, now I am the master”? No, were are missing a rage fit in which Vader tries to chop Obi-Wan and fails in-between.
The idea of Vader fighting Obi-Wan and falling into a lava-pit being the reason why he has the suit goes at least as far back as pre-production on RotJ (it's even in the novelization) and the notion of Vader's castle being on a lava world goes back to VERY early pre-production on tESB, so it's not something invented whole-cloth for the prequels. But yes, when ANH was developed the intention was VERY different.
For one thing Anakin and Vader were two different people, "Darth" was actually his first name, the Jedi purge was a more gradual affair with Vader pulling a Sauron, still playing the part of a Jedi so he could lure them one by one and murder them. So I think it's entirely fair to alter the context of those lines, since the later OT movies did that anyway.
Just for the sake of clarity though, let's actually take a look at what is said: -
Vader: I sense something. A presence I've not felt since . . .
Vader: He is here.
Tarkin: Obi-Wan Kenobi? What makes you think so?
Vader: A tremor in the Force. The last time I felt it was in the presence of my old master.
Vader: I've been waiting for you, Obi-Wan. We meet again at last. The circle is now complete. When I left you, I was but the learner. Now / am the master.
Ben: Only a master of evil, Darth.
Note that there's nothing there to indicate when they last met. That's just an assumption we've (quite reasonably) been working under. He doesn't say
"the last time we met I was but the learner", he says:
"when I left you . . .etc etc". That just means when he stopped being his pupil. Same thing for sensing Kenobi's presence; he didn't say when the last time was, just that it was the last time he was in his presence.
Honestly, as much as I would not have gone this way if it were up to me; there's nothing here that directly contradicts what's likely to happen on the show. Same thing with Leia's message; she doesn't mention the time they last met because it's not the point: she's not asking Obi-Wan to come for her, she's relaying her father's request for him. More than that she's not a girl talking to the hero that once saved her to come help her again, she a Senator addressing a legendary Jedi General, requesting on behalf of her father that he join the cause. So yeah, of course her language would reflect that. Leia is still a politician and a diplomat after all.