Distaff
My reading was the same as yours. As always the specifics of the case are where the problems lie. I am aware of another (similar) case where the individual got his SEP signed up during a MEP test but then forgot to ask for it to be signed off. He had done the one hour required with instructor within the required time period for the SEP but did not get it signed off. Before the SEP expired he asked for a sign off based on the earlier (multi) flight and was told he couldn't have it. The SEP rating then expired and he had to do a full retest (only a couple of weeks after having done the MEP test).
I think the sign off should have been done. I am not aware of a time limite for the sign off to be completed after the flight with instructor.