I don't think those words have the exact same meaning.
I don't think they can be exactly the same.
ーI don't think those words have the exact same meaning.
「それらの単語は全く同じ意味でないと思うんだけど」
the exact same meaning で「全く同じ意味・まさにぴったり同じ意味」
ーI don't think they can be exactly the same.
「それらは全く同じではありえないと思うんだけど」
I don't think they can be ... で「…ではありえないと思う」
ご参考まで!