No man, you shouldn't be 'extremely skeptical". You should understand and realise it's a guideline with a limited range of mostly general age ratings. As in, general guidelines you could consider but aren't a definite answer to whether something's appropriate.
I think some parents would agree that it may be better to wait until kids are a bit older with this Kirby title. Maybe not 10+ , but that's the lowest rank above E (which is supposed to also be suitable for young kids). The system needs to have a balance to not be too convoluted but still offer valuable information to parents. If there was a 7+ rating maybe Star Allies would fall into that category.
You should also remember that one element or one scene could cause the need to bump the rating up a category. It doesn't mean the whole game is unsuitable for children under 10, it means there are elements in the game that are not suitable for all children. Especially with the E10+ rating that might be a better description than what the rating suggests: E10+ is more like "slightly above E" than strictly 10+. I think keeping the E in the rating is a signal for that.
Yeah, and just like an MPAA rating is based on the current guidelines an ESRB rating is as well, those are also changing with the times. ;-)
I think the current system is one of the best and most balanced possible options. In Europe they have 3, 7, 12, 16 and 18. That might be a somewhat better ranging of the classifications.. Although for America I could see them just lowering E10 to E8 or E9 and keep Teen and Mature at age 13 and 17. But one has to wonder if such a small change is really necessary.