This is because essentially this is no longer TDM as we know it (where the Mac behaves like a TB-SATA bridge), but rather the Mac running a NAS service out of recovery mode. Essentially the Mac is now running an SMB server out of 1 True Recovery’s macOS. This is very different from the old TDM, where there was no macOS running at all and instead the SATA drive was just being shared over TB (or FW before that) as if the entire Mac were nothing but a bridge dongle (like this one). Of course to the user it should in principle make no difference, with a few clicks (instead of a boot and hold T) you should get fast access to the Mac’s internal drive. But what’s happening underneath is indeed quite different.
As so often, Howard Oakley has some interesting background on this new SMB target disk mode.