That has to be exactly the reason. C-suite Tesla people were stating on record the base 3 battery would be "less than 60 kWh" last year. Only thing that has changed was the Bolt's official EPA range rating, which blew away even the GM fans' expectations.
There would be no way Elon could release any version of a Model 3 with less rated range than the Bolt. They probably tried to match/exceed the Bolt's range with their original <60 kWh battery size, but found they couldn't, so they added the extra 5 kWh.
All Model 3 reservation holders wanting a smaller battery 3 should thank GM for forcing Tesla to up the kWh count.
edit: oh, also, many people also thought that the Bolt's 60 kWh battery size was the overall pack size, with less actually being usable. Now we've found out the usable capacity is 60, with the overall size being a few kWh more (64-65 kWh from owner estimates).