Why isn't there "is" in "They did it, thinking it more glamorous than..."?

Could you please help me with the grammar of this sentence? It's from an essay in a book on IELTS by Cambridge University Press.

People turn to buying the new brand from overseas nations, perhaps thinking it more glamorous than the one they are used to. As a result, local companies are likely to maximize their profits as they import foreign products.

Is it more glamorous grammatically correct, and if yes, why isn't it it is more...?

Thank you!

Thinking it [adjective] is grammatically correct; it's a shorter way of saying thinking that it is/was [adjective] or thinking it to be [adjective].