Perhaps he offers more immediate cover/ competition. I think, but could be wrong, that Matejo was expected to progress quicker than he did taking into account he is older than Bissouma who we are all expecting much of.
We bought Ales Mateo and it was expected that he would compete with Bruno and Schelotto for the right back spot but he has left so perhaps this Guildbert will replace him and with Balogun offer cover/competition for that spot