I hope this is true.
iOS13 would slow dows that phones.
And Apple needs to sell new phones!
Alternatively, they could simply hire competent programmers that can produce fast, non-bloated, properly multi-threaded code. Or give their current programmers time to optimize their code properly. But that requires the corporate mindset that they don't want to force old equipment into obsolescence unnecessarily, which of course they do want to do as long as it isn't blindingly obvious as that drives sales as a people's older device "get slow."
Never underestimate the power that bloated or otherwise crappy code has to cripple perfectly capable hardware. Clearest "sort of recent" example would be the BeOS running on old PPC hardware. The MacOS at the time often struggled to perform adequately under even the most basic of situations. Want to play a Quicktime movie and do anything else at all? Good luck! The BeOS on identical hardware easily provided 5 or 6 times the overall power. You could simultaneously play several QT movies AND have those projected and transformed in real-time onto a 3D floating cube, and the system would still be incredibly responsive. In other words, Apple's own software was catastrophically crippling their hardware.
VR gaming is another area where suddenly companies must, and are, producing much more optimized code, because they simply HAVE to do it until the hardware finally catches up. And, amazingly, they can! For those unfamiliar, a VR game with head-tracking requires 90fps to avoid inducing hurling on the part of the viewer. At the same time, doing true multi-projection 3D (as opposed to depth-map based 3D such as that performed by the SuperDepth 3D ReShade project) that VR requires often drops the FPS to one-third what it was without 3D. That is, a game that ran at only 30fps in the first place may suddenly run as low as 10fps once you transform it for 3D; both the GPU and the CPU take a huge-hit when doing true 3D. Now, take into consideration that 30FPS is "good enough" as a minimum when head-tracking isn't an issue and suddenly you have a situation where programmers may have to figure out how to get nine times the performance out of a game than they previously did (they've got to get their lousy, bloated code than can barely manage 10fps in 3D to run at 90fps as a minimum).
So in the end, if Apple actually wanted to support "older" devices, they certainly could. Every once in a while they might have to drop a feature that the hardware itself simply won't support, but they could definitely support far more hardware than they actually do.