if athletes consuming gu and other sources of non-solid nutrition suffered from severe dehydration and shutdown...I think this would be quite common knowledge by now

In fact this is quite common, but likes cramps is largely misdiagnosed. Bloated stomachs and GI issues are probably the most common problem for athletes of all levels, including pros, in an Ironman race. Try solids on a long training day and see for yourself, the differences are quite obvious.