It was originally from Australian demographer/writer Bernard Salt, a boomer, and it was intended to be mocking of himself and his generation [1]. But people missed the joke and got upset about it. It was Gurner who later promoted it as a serious thing as part of his real estate business marketing.
Hah! I just googled it because I'd seen "avocado toast" referenced enough times I finally got curious where it came from. I didn't realise there was another level to it.