The backstory presented in the text sounds a little curious since, since it infers "connection", as you point out.
Two practical reasons for not selecting the dead may be that:
1. By virtue of being dead, there is no more experience for them as that dead person... thus, the dead person cannot be subject to dukkha and cannot be receptive to metta. Thus, one would know on some level that metta towards the deceased is misaligned - for how can one wish for them to be happy? (Respect on the other hand, or wishing a favourable rebirth etc. would not be out of line.)
2. Recalling them to mind may bring grief, which is counter-productive to mental tranquility.
In the context of the backstory, if you were under the impression that the recipient was alive, that would seem sufficient as far as I can tell, though obviously not for the text. Taking a sutta perspective, mind creates all mental states (Dhp1-2) including metta... thus the actual object seems a pragmatic concern at best.