That makes sense. But where has Apple said it? I've only seen rumors from Taiwan saying that.
Yes so far it is only rumors, and the rumors are probably based on timing seen from M2 rumors. Kind of hard to extrapolate with a sample size of 2, especially when one of those is just a rumor itself! The startup of Apple Silicon designs may be at a different cadence than things go after they've got the first couple generations under their belt. Plus we can only see when things are released, we can't see when they were PLANNED to have been released.
Since Apple isn't announcing dates for this stuff if something slips by 3-6 months because they need to do a few mask revisions or whatever we'll see the timing and can say "they followed M1 with M1P/M1M by x months, and M2 by y months" but guesses on cadence based on that would be incorrect if the release dates had slipped due to mask revisions, covid issues, etc. or they were planned to be different at first as they got the Apple Silicon design pipeline rolling. With iPhone it is pretty obvious when something slips, because we know when to expect new models.
It will take a while to gain the same comfort level when new MBA or MBP or Mac Pro or whatever comes out - and it is possible they never set things in stone to the level they have with iPhone. And like iPhone, we very likely won't know (except via rumor) if the reason it is late has to do with the SoC or one of many other components. I mean, Steve Jobs famously delayed the white iPhone 4 by almost six months because the white in the home button didn't match the white on the case (different materials) to his level of satisfaction, so a delay could be for not only any reason you can think of but one you might never consider.
An 18 month cadence doesn't seem to make a lot of sense when the cores are on a 12 month design cycle. Though A15 doesn't seem have to got much than a couple minor tweaks and a process based clock bump compared to A14 so it remains to be seen if that's an aberration or they are going to move to a two year cadence.
Regardless of whether they keep the yearly cadence for iPhone SoCs or go to two years, splitting the difference at 18 months for Apple Silicon would be a really odd choice unless there were some parts of it that will be on a three year cadence. If they were basically saying "OK, M2 will be the first one to support 4x SoCs in a Mac Pro, and M3 will keep all the Apple Silicon only stuff like the interchip fabric pretty much unchanged and then do a ground up redesign with M4" then you could have phone SoCs at a one year cadence and major upgrades to the Mac SoCs every three, with an incremental bump with new cores and a process based clock bump at the halfway point so customers don't have to wait three years for a new model.