UEFI is the modern method for loading the operating system on the PC and provides enhanced security over the use of an old-fashioned BIOS. Standard Windows 11 installation requires the use of UEFI.
I suspect that TurboTax requires and checks for Windows 11 not for any security reasons but instead for software compatibility reasons, and does not check the boot method or for TPM presence. Maintaining compatibility with older OS versions can increase software development and maintenance costs and is probably the main reason for the Windows 11 requirement.
