He is saying, “idk what I saw but maybe it was something,” though. He's telling of his experience, he doesn't say "yes, they most definitely exist", but "I've experienced something that was nothing like I'd ever experienced and I know of no animal that could fit the experience I had." Him being a very experienced bushman brings quite a bit of credibility to that statement.

He's not challenging people on whether Saskquatch exist or not, he's challenging whether you think the multitude of people who have had such experiences and are sharing them with others, like him, are all lying about what they've experienced, completely fabricating a story of something that just happens to have commonalities with stories from others across borders and generations.