I can't be-leaf it's not flutter

[frindle]

Hello. Could you tell me what the underlined sentence means in this context?
I know it's sort of wordplay, but I don't quite follow it. Thank you.

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(In the story, "I" is a boy who is acting "Tree" in the school play.
And Flora is a girl who is acting "Leaf".)

"What are you doing after the show, Leaf?" I asked.
"I'm not sure, yet," said Flora, adding, after a moment, "But I can't be-leaf it's not flutter."
A play on the name of a well-known margarine product. It brought an appreciative rumble of laughter from the audience, who were starved of humour.

[Rover_KE]

There's a brand of margarine called I Can't Believe It's Not Butter.

If the audience laughed, they must have been desperate.

Rover