Apple didn't hack the Windows boot process on T2 Macs. They inserted an extra layer. The following is from memory and I'm glossing over and/or forgetting lots of details, but the broad strokes should be true.
When you turn the power on, at first the x86 CPU is held in reset while the T2 chip boots (using a derivative of iOS secure boot). Once booted, it verifies the signature of an x86 EFI firmware image in the T2 Mac's boot flash, puts it in RAM, and allows the x86 CPU to start its own boot process.
From then on, the x86 runs independently, treating the T2 chip as a collection of a few I/O peripherals rather than the center of the system. The most important peripheral in the T2 is the SSD, but others include the Secure Enclave (broadly similar functionality to a TPM), something for the Touchbar, and so forth. Many of these are similar to devices in standard PCs, even though not identical (e.g. the SSD is NVMe plus Apple extensions to NVMe).
So as far as Windows is concerned, a T2 Mac is a UEFI PC with some oddball peripherals which have vendor-supplied drivers. No kernel level changes are required.
This isn't true of Windows on Arm and Apple Silicon. Actually running Windows on AS is different than providing drivers for subsystems of AS.
Also, there's a de facto standard Arm system - this is being driven both by Arm laptop PCs and the emerging Arm server market. It's based on UEFI, and assumes certain base pieces of the system such as a more or less universally adopted Arm interrupt controller IP core. (Which Apple doesnt use...)
