I notice that too. Nomura himself stated that it's MX controlling Terra's body (or rather, the plot mysteries part of the old BBS Ultimania stated that).
But the dialogue itself is confusing. Most of the time it speaks as if it IS MX in Terra's body, but a few times it sounds like "Dark Terra" instead.
"You'll never be able to break these chains, they're OUR bonds!" (unless that line was meant to be mockery?)
EDIT: This confusion also happened in DDD, where the dialogue acted as if Ansem SoD was 100% Master Xehanort, but KH3 acted like he was separate.