That's how I would understand it as well.
Aethon nhw i Abertawe gyda'i gilydd "They went to Swansea together" -- the "they" is the nhw and the gyda'i gilydd is just "together", not "they together".
I would like to second the request that "together" be accepted as a translation for gyda'i gilydd.
Incidentally, shortly after I got this question, I got gyda'n gilydd, and there, "together" was accepted (though the "best translation" shown in the sentence discussion was "we together").
For consistency, please accept "together" also for this exercise. Thank you!