The question is what the priority is for the PR folks. I agree that they should be open with good, useful information if the Globe or B&T call them for comment, but I don't think the agency is well-served by engaging in a tit-for-tat argument on social media.
"You've been hearing a lot about these cars being out of service lately, here are the facts..." doesn't play very well with a public whipped up into a "crisis" frenzy. Better to focus on testing the fix. FWIW, the new train was queued up to leave the yard again just now, so they're working on it.
It's likely not entirely up to the agency either. T improvements, including the new OL fleet, are among the Gov's highest priorities. As such, his office will have quite a bit of influence over the messaging that comes out about this. Any response to press inquiries is likely vetted through the Gov's communications team before being sent back to the reporter. The T might (and probably would) agree that setting realistic expectations is a good idea. But that doesn't align with the politics of promoting the OL fleet replacement as a success today. For Baker, it needs to be a win now, not eventually down the road. So the T is caught between a rock (pressure from the Gov) and a hard place (the realities of launching an entire fleet of new train sets).