You are correct and I forgot about that. Obihai IP phones (and also ATAs) can be configured to bridge calls locally. Some other brands may do this. Also, with almost any IP phone, you can establish a three-way call (conference), which doesn’t involve the PBX, and the ‘transferor’ could then press mute or just remain silent.
Phones with a ‘transfer on conference hangup’ feature do perform a regular (SIP REFER) transfer when the user who established the three-way call hangs up.