Without seeing a lot more context, I think it's believable that this is indeed just the result of using Bootstrap.
Originality is vanishingly rare (if it exists at all, which I sometimes doubt), and it's certainly possible for multiple parties to independently end up with very similar designs, especially if those parties are all using common underlying tools.
"I visited the Twitter page to reach the people behind it when my heart sunk even deeper—I recognized them.
They were Cushion beta users.
Aside from feeling sick, I felt betrayed. Not only because they were Cushion users, but because I remembered answering numerous questions they had about how I built parts of the app. I was naive to assume they were just curious developers, but this community is built on sharing what we know."
Originality is vanishingly rare (if it exists at all, which I sometimes doubt), and it's certainly possible for multiple parties to independently end up with very similar designs, especially if those parties are all using common underlying tools.