Kya in this sentence

炭酸がとんでなきゃ大丈夫。。。開けてないって証拠。
-> If the soda isn't fizzing, then it's fine. It's proof it wasn't opened.

I still feel like this sounds kind of strange, even after asking 3 people... But either the writer made a mistake, or all of us misunderstood this sentence.
なくちゃ and なきゃ both mean the same, yeah? What is the right translation for this sentence?
Thanks in advance!

Yes, -nakya and -nakucha have the same meaning, and are used to add the meaning of "if not" to the verb that comes right before them. So in this case, とんでなきゃ would be the same as とんでいなければ, with theverb being とぶ.

However, I think that your interpretation of とぶ might be off, because the logic doesn't mesh in 「If the soda isn't fizzing, then it's fine. It's proof it wasn't opened.」. If soda isn't fizzing, that would mean the gas has escaped, which would only happen if it HAD been opened.

As with a lot of Japanese sentences, it's hard to be 100-percent sure on the meaning without the specific context it's being used in, but I'd interpret
炭酸がとんでなきゃ大丈夫。。。開けてないって証拠。
to mean,
"If the carbonation hasn't disappeared, it's OK...That's proof that it hasn't been opened."
or, to put it more naturally in English
"As long as it's still fizzing, it's OK...That's the proof that it hasn't been opened."

とぶ sometimes gets used to mean "disappear" (as an extension of its primary meaning "fly" also being used to mean "fly off somewhere"). 炭酸 actually refers to the state of carbonation itself, not the carbonated beverage.