Uh... M1 (at least when Mac-fused) isn't actually locked-down. You do have to jump through a few hoops to boot modified kernels[0], but it's a reasonable level of security for a chip primarily intended to go into laptops. There's an ongoing effort to reverse-engineer the M1 and port Linux to it[1], which has actually gone really well.

There's also plenty of ARM hardware out there that really doesn't bother with lockdowns. Raspberry Pi, for example, will happily boot whatever you put on the SD card[2]. There's also plenty of other dev boards and chips that are similarly not locked-down. The reason why people think of ARM as "more locked-down" is because it's more often used in locked-down hardware, but there's no architectural requirement for it to be this way.

The real big problem with ARM is that every chip is very different from one another. SoC designers have a habit of reassigning addresses and IDs on every new chip, with no thought towards backwards compatibility. So even if an ARM chip has no technical protection against third-party operating systems, any Linux kernel you build is unlikely to boot unless the SoC vendor has bothered to upstream support for it. Ironically, Apple is one of the few ARM vendors that actually does maintain backwards compatibility where feasible.

[0] The first admin account is granted possession of a local signing key that lets that user sign kernels. This means that you need to be able to log into that account under macOS in order to install Linux - evil maids need not apply.

[1] Legend has it that if you say "M1" three times in a HackerNews comment, you'll summon the primary developer of Asahi Linux.

[2] You do have to provide a GPU boot blob; no clue if there's a project to reimplement that.