No - you have fallen foul of Gaelic grammar. Feminine words slenderize in the dative singular. Because they want to introduce some interesting things before they have covered all the grammar, they specifically mention this word in the notes (more to stop people complaining than because they want you to learn it). So just accept it for now and learn about it later.
This is of course the natural way to learn any language. Kids learn what is right without even questioning why a word is sometimes one way and sometimes another. So now you have the opportunity to do the same. Except you don't because your adult brain starts shouting at you "Why has this word changed?"
People will argue between
- he walks as slow as a snail
- he walks as slowly as a snail
but no one can dispute that you can say
- he is as slow as a snail
I think this is the more normal thing to say. As far as the grammar is concerned, note that although there is the word gu in Gaelic that translates '-ly', it would be suppressed here by the cho, so you cannot actually tell from this Gaelic phrase on its own whether the slaodach is an adjective or an adverb.