Have you actually tried? Personally I have evaluated and then abandonded clang-format (for reasons including but not limited to the interplay between macros and indentation), and most tool's support for tab-indentation + space-alignments is flaky to non-existent. I wouldn't want to constrain my setup to one that integrates clang-format just for a needlessly complicated requirement when I could just abandon tabs altogether.
If you want to keep space alignments that's honestly to be expected. Exclusively using tabs would work much better, partly for the reason you have said. On the requirement of clang-format itself though... Yeah, that is more like a problem of C and its ecosystem indeed.
