Your discussion with Dave including especially the Secure System and Hyper-V virtualization fit with other discussions we've had here recently with others asking questions about the Core Isolation and Memory Integrity features of Windows 10.
Since enabling Memory Integrity requires that virtualization be enabled so that a Hyper-V virtual machine can be opened to contain the core Windows operating system components, the addition of a Secure System service to monitor this process seemed reasonable as well.
Enable virtualization-based protection of code integrity - Windows security | Microsoft Docs
Since I've got this operating on my own Windows Home based Microsoft Surface Go tablet running in S Mode, I looked and found both the Secure System and Lsalso.exe processes running on this system as well.
I checked for the Secure System process both before and after disabling the Memory Integrity option in Core isolation found under Device security in the Windows Security at a glance console and it disappeared and returned as expected. However, I forgot to look for the Lsalso.exe process before returning Memory Integrity to enabled, so you'll need to test this yourself if you want to be certain this is related to the same setting as well.
You'll notice under Windows Security - Device security - Core isolation that Virtualization-based security is specifically mentioned, so the above all makes perfect sense.
So you two were very close, just hadn't made the final leap to the primary reason behind these virtualization-based elements of Windows 10 being enabled on any system capable of operating with Memory integrity enabled.
Rob