It's not excessive. In no way were Lexus going to start suddenly using the Mazda inline-six engines after there were leaks talking about their power figures, which was less or right around where Lexus' powertrains already push out in overall output.
Speaking of platform, Lexus would either do the unthinkable and convert all of their vehicles to TNGA-K, or they'd jump to EV's quicker with the eTNGA platform, or they'd actually build a TNGA-L IS, which was next to impossible due to packaging issues. All of these were more likely than a Mazda platform.
And YET people kept believing it.
Well the rumours I remember was saying it would be a collaboration platform and collaboration motor, obviously not blatantly putting a Mazda-spec motor into a Mazda platform. Obviously the final products would've been way more individualized and effort put into if it were true.
Given both the true next gen IS was cancelled early in favour of the facelift as well as the Mazda6 sedan, we'll never know if there was a collaboration effort, not to mention Carmaker1 had previously alluded to there being a potential partnership for this, before it was known to be cancelled.
There have been NUMEROUS rumours out of Japan about the possible Mazda/Lexus venture in 2018-2020, clearly there was a basis to these otherwise there wouldn't be as many reports on this, nor supporters of this in the forums like CM1.
Either way we're way past that, and we've got to see what Toyota's going to do for their future i6. I STILL think there is a possibility they will use a reworked version of the Mazda i6, one that they build off of if they do collaborate on it, or slap two G16s together or something. Who knows? Either way, it's not very far fetched at all.
EDIT: And yes, at the end of the day rumours are rumour, but there's so much that happens behind closed doors and get cancelled that you can never really know what is true or not. Unfair to undermine those who actually believed these rumours, as there was a strong basis for it when it came out.
