"It is definitely a name."
In a realm where there was a government-approved list of names parents were allowed to give their children, but someone had a name that was not on the list, and I had figured out that the unfamiliar, non-approved sound combination was indeed their name, I might say pongna' 'oHbe' 'a pong 'oHbej. In any event, the course accepts both pongna' 'oH and pong 'oHbej and rightly rejects 'oHbej pong.
C'mon, I'm not saying the sample sentence is wrong or unapproved, just that using a normal tool to de-emphasize the "be-ness" of the sentence in a language with no verb "to be" is a better style.
I reject your distinction between the two sentences. -na' says that the identity of the noun is known by the speaker to be correct; -bej in this sentence says that the identity of the noun (the "to be" sentence is a sentence of identity) is known by the speaker to be correct. They're the same.
I also have no way to see which sentences the course accepts — I didn't say it didn't accept the -na' form — and I wasn't replying to the erroneous 'oHbej pong.