using two different sessions from two different computers I am able to use the same code in both sessions
TOTP is time-based one time password. Oversimplified, it's just a hash of the current timestamp and the secret. So if two (or 200) devices generate a token from the same secret at the same time, the token have to be the same.
Their response was that they don't implement OTP, only 2FA.
And that means they have no idea what they are talking about. OTP is a form of 2FA. Like iris scanners, fingerprints, voice print, secrets printed card...
They let you login with two computers at the same time because they don't employ a single-session policy server-side. As long as the credentials are valid, they allow it. Just like the vast majority of sites out there: Gmail, Facebook, Amazon, Twitter... Some banks don't allow that due to "security reasons", but allowing multiple sessions is the default behavior almost everywhere.