well lets say its not the most unintuitive thing ever.
but it doesnt show how to make them change in an animation.
Not directly no, in tutorial do this do that fashion*. Theres a general discussion on how to key ANY of theese multi attributes, so their demontsrtion is not about blendshape weights but a comparable weight attribute. And they tell you that up front. So its assumed that by the time you know how to paint the weights you know how to key them. On a related note they also discuss how to use the cache to accmplish the same thing. (cache is better if you intend to paint frame by frame)
so the simples brute force way of
setKeyframe blendShapeName.inputTarget[X].inputTargetGroup[X]
isnt really rocket science now is it.
So its a 1 - 1 mapping, yes you can use same for cluster weights, vertex colors, hell you can even key per particle atrributes etc. But kudos for you you can spot things that ought to be self explanatory to most users but do not get it anyway. So Im saying your perceptive but not more so than the avarage user SHOULD be.
But yes no go and do anything you wish with the nodes you deciperewd the first key.
There used to be no sane way to do this but that was a looong time ago.
BESIDES: any attribute in maya that is manipulatable by a user can be animated via a expression node even if its not possible otherwise.
- the manual would be about 1000,000,000,000,000 pages long if it did that to ALL features.