The blocking people asking about it on Twitter is what leads me to be concerned. While there may be an explanation, blocking third parties trying to get your side of the story seems to imply there’s something they feel they need to hide from.
> If there is another side to the story, whats taking so long to tell it?
To play devil's advocate, they need to be able to respond to the accusation well. They can't just jot something down and fire it off, right? They're starting off in a defensive position, and need to address the accusation in away that paints them as the good guys (which they might be).
To form a response to Upstart, yes - but not to form a response to this blog post. (Which, to be fair, as a PR company they should have known was coming, and should have had contingency planning to deal with this.)
If you have no response, it's acceptable to write "We are deeply concerned about this situation regarding our former client, and will prepare a more comprehensive response to be publicized later".