I think if anything the CP being that large was niantic not wanting to make the rocket stuff too easy, since it would give people a reason to go out, look for what beats what typings, and start making pokemon good for battles.
if we have the rocket grunts be around regular CP, no stat changes, that wouldn’t encourage people to learn about how battling works in trainer battles, or be more inclined to start learning about it, something that would make sense given GBL is a thing they likely had planned for some time, get people interested in battling early, and maybe making one or two pokemon for higher level battling before the format got here and no one had anything usable prepared, especially for things like masters where things are maxed out
Could it be the shadow being super effective thing? Maybe, but the stat changes weren’t specifically attack, it was all the stats were inflated, so you needed to give thought to what to use and what pokemon were good for battling in trainer battles. Yes things like PuP spam or other moves liek that for stunlocking worked on most battles, and a few other things make the system imperfect, but I dont think its specifically the type effectiveness, it felt like a soft introduction to PvP