One thing that seL4 requires is an mmu in order to support the isolation claims. The intel me type cpu would need to have some of these features that micorcontrollers don't usually have.
That's not necessarily true. There are VMM or hypervisor projects that utilize seL4 for x86 and ARM [1][2][3]. In this situation there isn't really one that is in "control". You could also have other threads or components that have higher privileges that can maybe do other monitoring or control activities.
It does say this: The hypervisor extension has been designed to be efficiently emulable on platforms that donot implement the extension, by running the hypervisor in S-mode and trapping into M-modefor hypervisor CSR accesses and to maintain shadow page tables. The majority of CSR accessesfor type-2 hypervisors are valid S-mode accesses so need not be trapped. Hypervisors can supportnested virtualization analogously.