Depends on which feeder level they took the transformer from, and at this level of KW draw, a higher level feed, if available, makes sense.
PECO offers supply voltages of 13.2kv, 33kv, 69kv, 138kv, and 230kv. So I suspect the 35kv is a typo for 33kv, or a rounding of the person saying it. I'm not surprised about a 33kv feeder for this size transformer as that would be a large number of amps at 13.2kv. (Actually I'm sort of surprised it's not a 69kv feed, so I guess that wasn't available, most of the switchgear I've had to work with in a past life was off 69kv PECO feeders.)
Having said that, I'm sure there are still some weird old one-off's in substations in PECO land, as they still even offer 2 phase power in some places in center city (and I don't me split single phase).