It means nothing's finalized because the Type 10 RFP is nowhere near ready. The trainlining electronics on the Type 9's was revamped to get rid of all legacy analog cruft and employ software abstraction for easier forward compatibility. For the 10's they either could have mandated plug-and-play compatibility as a requirement by having the bidders build to the software, or just included a feature preference for it as a scoring variable for the bidding to see what the bidders offered before making their stance. They started out leaning towards the former earlier on in the study that became GLT...now seem to be leaning towards the latter. And it might swing a couple more times before anything is put out to formal bid.
At any rate, it doesn't mean the 10's won't be trainline-compatible just because they don't have a binding mandate to be so. Since the newest makes are all very software-heavy it's not a big technical hurdle to build in the software abstraction layer for MU'ing with a Type 9. Bidders can assess technical feasibility and costs involved and offer it as an option. If they say they can do it, there's no reason for the T not to opt for it as a feature because universal trainlining compatibility is an asset. Not mandating it yet means they're just casting a wider net for potential makes and builders, and haven't yet drilled down far enough to decide whether this is a feature worth thinning the field over.
BTW...even if a handful of cars go to Mattapan, at least two-thirds of the fleet is staying on the Green Line. Nothing really changes on the fleet management side.