この英文のasの役割について

英語の文がおかしいです。直します。

They will not be as fatigued, stressed, or anxious as they were before.

「…as 形容詞 as …」の比較級です。同じ意味で、次のようにも書けます。

They will not feel as fatigued, stressed, or anxious as they did before.

They will feel less fatigued, stressed, or anxious than they did before.

形容詞でなくて名詞を使うなら次のようになります。

They will not experience as much fatigue, stress, or anxiety as they did before.

Their fatigue, stress, and anxiety levels will be lower than before.

They will not be as fatigue, stressed or anxiety as they were before.
という文の１つ目のasはその後ろに続く形容詞にかかる副詞です。
通常 as + 形容詞や副詞とくれば、次に接続詞の as を置いて主文と比較する内容の節が期待されます。