Ok but I think the point is still the same. Carpenter might not have had the same thing Lancaster had in mind. So imo Lancaster's thoughts on his scripts ending, isn't necessarily the definitive answer on whether or not 1 or both of them (Childs & Macready) were infected in the version we see.
I'll agree that Lancaster's take is not the definitive take.
But if fits in line with Carpenter's narrative structure of the film. The ending's nature wasn't to question MacReady/Childs, that's a byproduct of the ending. Carpenter and company themselves didn't fully understand the assimilation concept, and there are conflicting tidbits of information during the film that add a layer of confusion to the narrative. This actually helps the film in a weird way.
The ending highlights the futile nature of their efforts and their shortcomings. The Thing doesn't assimilate them in the end, but they're no better off, because of their inability to trust one another. They're both going to die. That's the depressing nature of the ending.