I was listening to twice's more and more japanese version and they sing:
Is it perhaps an abrreviated form of そうにない or some other grammar point? What's the explanation for this?
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Maybe poetic license? The lyrics is a mixture of English and Japanese. It looks as though the author/translator preferred to keep the number of syllables over grammaticality. https://www.uta-net.com/song/287624/
止められそうにない would be more correct but you can get away with skipping に in a song.
When you want to shorten it, you can also use 止められそにない. I'm not sure if it would have helped in the song, though, as it wouldn't have fewer syllables (while it would have fewer moras).