> A Linux USB most likely requires Secure Boot to be turned off to boot
That hasn't been my experience. All the recent laptops I've owned (Dell and HP) had a default secure boot setup that allowed booting to Ubuntu and Fedora without disabling Secure Boot. In fact, nowadays even Ventoy works with Secure Boot [0], and I've managed to use it with the setting enabled on all machines I've tested, however in this case you might need to enroll the keys on the first boot, which I imagine will trigger BitLocker.
Apparently what happened is that Microsoft now signs some third party certs for common Linux distributions, and some setups allow these to boot by default. However, it also looks like Microsoft wants these certs disabled by default [1], which should improve BitLocker integrity on average.
Although I believe what happened in OP's situation was that BitLocker wasn't actually enabled or working, likely due to misconfiguration or lack of any.
That hasn't been my experience. All the recent laptops I've owned (Dell and HP) had a default secure boot setup that allowed booting to Ubuntu and Fedora without disabling Secure Boot. In fact, nowadays even Ventoy works with Secure Boot [0], and I've managed to use it with the setting enabled on all machines I've tested, however in this case you might need to enroll the keys on the first boot, which I imagine will trigger BitLocker.
Apparently what happened is that Microsoft now signs some third party certs for common Linux distributions, and some setups allow these to boot by default. However, it also looks like Microsoft wants these certs disabled by default [1], which should improve BitLocker integrity on average.
Although I believe what happened in OP's situation was that BitLocker wasn't actually enabled or working, likely due to misconfiguration or lack of any.
[0]: https://www.ventoy.net/en/doc_secure.html
[1]: https://www.omglinux.com/boot-linux-modern-lenovo-thinkpads-...