If a Guide is able enough, couldn't they also take on one of the other roles? I think that's how the rules work.
That is explicitly how the rules do not work; see p 109 "Assign Journey Roles".
It says that with more than four you can double up, and of course with less than three you have to double up. It says that you can't double up on the Guide. It doesn't say that the Guide can't take on other roles.
Here is the relevant section. The English could be clearer, with more examples, but I think that's what it says.
A travelling Company must cover all four available roles. If there are more than four Company members, more than one Player-hero may be assigned the same role — with the exception of the Company’s Guide (there can be only one Guide, but more than one Player-hero can stand as a Lookout or Hunter, etc.). Conversely, if the Company counts less than four Player-heroes, then one or more adventurers will have to take on the duty of multiple roles (still, only one Player-hero can be the Guide).