GP was not well-phrased. I think they meant "the first detection of a MACHO candidates could be via a WIMP detection" (a black hole planet 9 would likely be in the last uneliminated sliver of MACHO candidate space and we'd see it via WIMP annihilation)

IIRC, the other way around. The editor who greenlighted the WIMP paper for print (and only because it was already an established name) regretted this decision once he saw the MACHO paper. Apparently he doesn't like cute names.