I think that 'the involvement of' in this case may mean 'changes to' and since Product
is not modified only one AR is affected. I actually read this bit last night and just a bit further on he does mention that one should really aim to only manipulate one AR per transaction. So it seems as though there will be exceptions.
It isn't a bad idea to only manipulate a single AR per transaction and typically you'll find that you end up with such a design anyway.
There are always going to be exceptions to the rule but those should be carefully considered.