They are saying that not knowing the DC of Aid Another, combined with a penalty for failure, is going to put them off trying it. And that will mean that Skill Challenges will end up excluding some players when they don't have relevant skills.
This is a misnomer created by having too much transparency in SC. If you reveal a SC in full, the only decision a player need make is matching their trained skills with the best DC. Its a purely mathematical exercise of obtaining the best chance of success.
Not only does this effectively remove all creativity from the SC but also the maths behind SC just can't handle that level of transparency and the DC prove pitifully low.
If you use less transparency (such as just revealing that a SC has commenced and Complexity) then it less of an issue. The reason is that the players are then trying to factor two things:
1. Trained Skill. They know they get a +5 to this roll, so they will gravitate towards them. This is a good thing.
2. Low DCs. An Easy DC is the equivalent of getting > a +5 over a Med DC and >+10 over a Hard DC. If you look at it like this, working out what's an Easy DC is actually more powerful than choosing a trained skill.
As such, a GM must really focus on presenting the second option as a viable and powerful one in a SC. The reason for this is that it is the second that changes a SC from a simple mathematical exercise to one of creativity.
As a GM, I tend to award narratively appropriate solutions with low DCs. That has the effect of focussing the players on listening and taking as much note to the narrative as their PC's skills.
But choosing a trained skill is much less risky than trying to determine what's a low DC, you say. Possibly true. However, it need not be so. Auto-failures on inapporpriate Skill use or other negative consequences add back an element of risk.
If you take this approach, your SC won't become about PCs just trying to use their best Skills whilst those with semingly inappropriate Skills are left on the sideline to Aid Another. Instead, every PC no matter what their Skills can have a decent chance of success if they listen to the narrative and come up with decent ways to resolve it. If they manage to do so, whilst sticking to the Trained Skills, good for them.