How grammatical is “not only that, but there's more”?

Question

A website lists some activities offered in a photography tour, then adds Not only that, but there's more.

... [S]unset and moon rise occur at the perfect time and will allow us to photograph New York Harbor during the day and the New York skyline at sunset, the blue hour and at night from the incredible vantage point of Liberty State Park and the Empty Sky Memorial. Not only that, but there’s more. There will be a full moon rising shortly after sunset... .

http://www.adventures-in-photography.com/meetup/photograph-ny-harbor-by-boat-the-statue-of-liberty-the-ship-graveyard-and-more-3/

Is the sentence grammatical or generally acceptable?

I think but should introduce a contradicting idea, but it doesn't do that here. I think the but should be removed and replaced with a comma or a dot. Do you agree? Why?

Answer 1

But, in such constructions, does introduce a contrast.

It introduces a contrast with a supposed idea that there is only that.

Although the literal statement is negated ([there is] not only that), the negation implies the idea that someone believes or may believe that there is only that.

The contrast is between what may have been thought or assumed (there's only that) and what's actually the case (there's more).

Logic in language is often obscure like this. English isn't math or a programming language! It wasn't created by conscious design.