Thanks but as my understanding is, for issue of the ATPL you must have 100hrs PIC (gained from ppl/cpl training) and a further 150hrs of PICus - total 250hours.
And i doubt there are many FO's at 1500 hours going for command upgrade therefore logging the time as you say, however plenty of people are getting their ATPL issued with 1500 hours...
What you say makes sense of the expression PIC under supervision though!