If you turn off quiet boot, you'll see an impressive list of things getting started up, many of which the kernel isn't in full control of. DHCP (sometimes), gpus, NTP, disk decryption, TPM, monitor probing, USB hub enumeration, on and on.
This could be a very, very long list of hypothetical changes :)
Presumably the dock can be detached while booted, so the kernel should also be able to handle booting and only detect at some point that the dock has disappeared
Presumably, most faster boot OSes simply don't support as many devices. Amazon was able to get Firecracker to boot Linux damn near instantly by pretty much removing all hardware support from the kernel since they know for sure it's only ever running as a VM on a hypervisor they strictly control and won't change.
If you know for sure the devices you need, you can compile the kernel yourself and remove all the stuff you don't need and possibly even get rid of udev and just hardcode what it normally detects.