Yes it is, they are the only supplier on the market for MacOS hardware. In the market for Linux or Windows, hardware is priced at a competitive level.
You may argue that the relevant market is for "computers" as a whole, however it can be argued that bundling hardware and software to charge high prices is a classic monopoly behavior nonetheless.
Companies cannot be forced to sell their operating systems distinct from hardware if they choose not to. That's a ridiculous expectation. There is no market for macOS, only Macs, and that market is the broader PC market, which Apple is nowhere near monopoly power
Saying Apple has a "macOS hardware" monopoly is like saying Dyson has a "Dyson motor monopoly"
Companies get to choose what their products are, full stop
MacOS is also an operating system or a platform. Tesla isn't really a platform.
But... funny you brought up Tesla, because Tesla also had this exact problem! Tesla had the supercharging network, which they own and manufacture. But superchargers aren't just a product, they're a platform.
Tesla had a monopoly on superchargers, until they pre-emptively opened up the network and open sourced the connector. If they hadn't, IMO it was extremely likely they would've been forced, eventually.