OLC comment, MOP logging deactivated
Sorry, I paraphrased too briefly, but the net effect of the code is that FES typically doesn't meet the noise threshold and that MOP is therefore necessary, per your last statement.
As Eric asked above, will ENL "hear" an FES that is running at low enough power to double your L/D or reduce your sink rate in a thermal?
Anecdotal evidence suggests that it can not and some electric engine gliders have been seen assisting themselves with power that is not being recorded in an IGC file by ENL.
However, thanks to @kinsell for reading the OLC rules we now know why OLC is rejecting FES claims without MOP.