This question is prompted by a particular passage in Storm King's Thunder:
In Storm King's Thunder on page 125 the heroes face a problem and the book suggests they might solve it by polymorphing a hero into a giant. They are level 7 at this point.
The level 4 spell Polymorph allows to transform a character into a beast with challenge rating ≤ character level and Hill Giant and Stone Giant are CR5 and CR7 respectively, so I allowed it. I only later realized that giants aren't beasts, so with rules as written in the Player's Handbook you'd need True Polymorph to polymorph into a giant. BUT, that's a level 9 spell which the players will never get in this adventure because it ends at level 11. So I'm thinking I must be missing something here.
My players now want to know whether polymorphing into a giant works or not.
So my question is: Are there any errata or author's comments or other official statements indicating that you can use the 4th level spell Polymorph to polymorph into a giant, at least in this case, or that this was a mistaken suggestion that shouldn't have been in the adventure? Please cite explicit quotes from designer statements!