Why "It isn't teddy bear" is correct?

[seito]

I listened to it a few times on my tablet without headphones and I'd say that she does say the "a"... But I'm not so much hearing it as picking it up from the prosody of the sentence.

[Lawyer&Mom]

I think she absolutely says “a” Teddy Bear. But, I’ve had lots of exposure to British English which I think is helpful in this case.

[Ani]

I hear the "a" easily.
The obvious solution is to switch to listening to American English where they'd be no risk of double enunciated "t"s, and the expiration after a "t" sounding like another word. Both cases would only have a single /t/ sound for Teddy
"It isn't Teddy Bear" would have a nice word like "itisin" and "it isn't a teddy bear" would get it's own word "itisinnuh"

[kanewai]

I absolutely do not hear the 'a,' with or without headphones. This reminds me of the "what color is the dress" meme. If I heard the sentence out of context I would have thought it was a proper name, i.e. "It isn't Teddy Bear."

What accent is this, anyways? I know it's some kind of British accent, but there seem to be so many for one small island.

[Cainntear]

I'm on the side of "a" -- it's very heavily reduced, but I reckon it's there. Splitting Ts isn't something that's generally done, even in careful speech. Moreover, the way she's pronouncing "teddy bear" sounds to me like a compound noun, not a name.