That is what I heard too. Right now as a Mariners fan the return we got for Segura looks bad. However, I’m not sure there was a huge market for Segura. One, there is the stipulation in the contract you mentioned. Two, MLB.com(I think) wrote a good article showing there probably weren’t many teams who would be looking for a SS that wasn’t named Machado. Three, the Phillies probably knew there wasn’t a big market for him and that the Mariners really really did not want him on the opening day roster.
Segura struggled the second half of the season. If you bring him into the season and with the intention to trade him at the deadline there is a lot of risk that he will continue to regress or get injured. If the Mariners were determined to trade him before the season starts, this probably was the best they could do.