Virtualization isn't terribly hard to add to a core, so they could've started thinking about it at any time. It's possible that some of their cores already support it and we just don't know; that would be a very Apple thing to do. The way virtualization on ARM works (at least the way ARM themselves implemented it; Apple could've done something differently) is that there are three execution levels: EL2 (VM), EL1 (guest kernel), and EL0 (guest userspace). So a device that supports EL2 but drops immediately to EL1 on boot to run a normal kernel (without virtualization active) would not necessarily have obvious "tells" that it supports virtualization, unless you broke into the boot process early enough to catch it in EL2.

It would be interesting to break into an A11 device using the checkm8 exploit and see if there is any evidence of EL2/virtualization support on that core.

Here's a fun one though: Apple CPUs did at least at one point support EL3 (that's one level higher, TrustZone), which they used for KPP:

https://xerub.github.io/ios/kpp/2017/04/13/tick-tock.html

Which suggests they might support EL2 and virtualization too. Honestly, I can't find any trustworthy reference claiming that existing Apple Silicon supports virtualization, nor that it doesn't. For all we know it does..