If you hold a pilot certificate and get a PIC AMEL endorsement, you can act PIC and log multi PIC time solo.
Do you have any documentation that confirms this? You can do it solo that has already been covered, you cannot log PIC in a multi without a multi certificate. This endorsement cannot be applied to that, unless you have some documentation I, and I suspect most of us here, have not seen. It would seem stupid to have an "acting as PIC" stipulation in the regulations for the purposes of multi training if you could just use this endorsement, wouldn't you think?