I, too, am not a lawyer. the specifics of plagiarism are settled in court.
Xenonauts (that's what you mean, isn't it?) is quite close, but probably different enough to be considered a "spiritual successor" or whatever you want to call it. they are not using graphics that look like x-com, they are not using x-com names, some game mechanics are different...
but that's not what openxcom is aiming for, is it? it is trying to mimic the original. the defined goal for v1.0 is to have a game that looks and plays like x-com. this will always be considered plagiarism unless it requires the original, don't you think?