The title is bad, but...
> Chris Belczynski, an astrophysicist at Warsaw University, previously felt so sure that such a large specimen wouldn’t be seen that in 2017 he placed a bet with colleagues. “I think we are about to lose the bet,” Belczynski said, “and for the good of science!”
LIGO has existed for decades and by 2017 had already detected a merger. My guess is he considered black holes in this range rare enough that we were unlikely to detect any. If he believed they were impossible the article certainly doesn’t elucidate on that fact.