Hi @Gualandi0
Welcome to the Community!
Thanks for your question, but I feel a little clarity is needed here.
Yes - the 3.5mm to USB-C cable that comes supplied with the Ace headphones can be used for a line-in connection to an Era 100 (or Era 300, or Move 2), bypassing the need for a Sonos Adaptor.
In addition, the Sonos Adaptor can be used as an (unrecommended) alternative to the Ace’s supplied cable (though a 3.5mm to 3.5mm cable would be needed too).
As the others have rightly (but superfluously) stated, you cannot use the cable to listen to content the Era is playing on your Ace headphones.
I hope this helps.
"Common sense is the collection of prejudices acquired by age eighteen." - Albert Einstein