Ok Im going to reply to myself. This is something I should have checked before.
Libelium's module for Sigfox is a TD1207
MKRFOX1200's module for Sigfox is a ATA8520
I've read the datasheets for both. TD1207 mentions in each "send" command the ACK bit that can be set or clear so you await for a downlink "ack" after sending
ATA8520's datasheet doesn't mention this functionality at all in it's documentation, nor even contains the word "ack" in the whole document.
So now I KNOW that there's no way to send with NO ACK in a MKRFOX1200 board
Thanks