I'm more interested by the fact that it apparently didn't work this way before Tahoe.

IIRC there are options for exportable and un-exportable private keys when you make one in the secure enclave. Going to guess Tahoe made them un-exportable by default.

	▲	what 35 minutes ago \| parent [-]
		As far as I know, you have never been able to import or export keys from the Secure Enclave. It’s more likely that previously keys were stored in the keychain and now they are generated in the SE by default.