Hi TyroPicard,
The ECB and APU starter are fed by the battery bus, which is not powered in emerelec config.
In fact, if you check the table of the systems remaining in emerelec, both on the QRH or the FCOM 3, you will see that with the EMER GEN RUNNING you can start it, but with BAT ONLY it is Inop.
I agree these manuals often suck, because they are made by engineers and not pilots...
They assume one must clearly see an important point like this one simply because it's in the table, instead of clearly stating so, in a more "pilot" way....
Please let me know if it's me who have missed something (quite plausible as I'm rather new on the Bus...)
LEM