That assumption is not necessarily true.
What this implies is that there is a market of existing consumers that would not buy an iPhone because it lacks OSX support.
The iPhone portion of Apple's business generates around $144B in YoY revenue in Q1FY27 [0].
Whenever an organization contemplates building a net new capability like the one you mentioned, a quick test is whether it would be able to generate and sustain at minimum the equivalent of 1% of yearly revenue.
If this was a $1B revenue opportunity it would have been implemented, but it's not.
Nor is it a feature that can actively or dramatically increase Apple's market share in most markets.
A good proxy of such demand would have been a sudden increase in iOS users using USB-C screen share and a Bluetooth keyboard to interface with an iPhone in a desktop form factor (something which has been enabled since iOS 15), but such an increase has not happened.
[0] - https://counterpointresearch.com/en/insights/apple-reaches-a...