They are Xenophile FE, which uses tachyon lance and mix of kinetic/energy weapons. I don't think shield would do better against Materialist FE which uses disruptors.
Disruptors are useless anyway. Spiritualists are the only FE with decent weapons. Didn't know that xenophile where also basically kinetic only.
I tried to reproduce you results but my 50 battleships lose to this xenophile fleet. How many repeatable techs did you use? Anyway, in dueling battleships, the decoder is certainly not OP:
Pretty much equal on armor ships vs +10% hull and cleary weaker on shield ships vs shield capacitors:
Remaining: hull, decoder:
3,0
0, 10
7,0
0,3
9,0
Remaining: capacitor, decoder:
8,0
6,0
9,0