Yes, those wingtip units are combustion heaters.
Interestingly, somebody on the WIX forum, where I also asked this question, said the C-124's APU was a pair of 50-hp V-twin recips in the two outboard nacelles, and he was quoting from the three-page section on APU operation in his father's C-124A POH, which he had saved. So I guess the answer is that SOME C-124s had turbine APUs...