The only thing to support that idea is Corocoro grouped the four together and asked what the reader thinks their evolutions will look like, but it was also the first time the starters were shown in Corocoro and it's possible they just grouped them because they all have notable evolutions, the starters being starters and Rockruff having its split gimmick. It's also possible the only other Pokemon in the issue, Komala, just doesn't evolve. Or it's possible those four all have some sort of visual differences in their evolutions somehow, though it's also possible the ones the starters have aren't quite the same as Rockruff. Maybe Synchro is actually a thing and all three starters and both of Rockruff's forms get them.
We just don't know. Personally, I think they won't, but we'll see.
