I'm using some because the parent used sum. And s/he's using sum because s/he is emulating tye's addition of booleans.
You're right that !! screams boolean. However, that's not what is happening here. We are adding numbers here. Let me ask you what "true + false + true" is. That's just nonsensical. Programmers have been getting away with it for years because computers only deal with numbers  whether that's boolean, character, or, oddly enough, numerical. But, abstractly, it makes no sense. You can't add booleans. Boolean algebra consists of operators such as not, and, or, xor, nand, and nor. (I may have missed some  it's been a few years since I've dealt that directly with boolean algebra in the theoretical sense.) Addition, subtraction, multiplication, division  all of these are nonsensical in the boolean world. In fact, the multiplication symbol (the middledot, ·) was reused for the "and" operator, and the addition symbol (+) was reused for the "or" operator, IIRC (which I may not recall correctly...). And, as in mathematical algebra, the multiplication symbol is optional between terms, further reusing the sigils.
Therefore, adding boolean values is just confusing. The answer to the question above is that "true + false + true" equals "true" (numerically: 1  0  1)  and the OP wants this to be a count of 2. Which, if my memory serves, the OP wants "a nor b nor c" Unfortunately, the nor solution doesn't scale past three.
grep, unlike sum, uses boolean context. So the choices are to either sum numbers (using the ternary operator, for example), or to grep a list in scalar context. Adding booleans makes me question whether the author knows what s/he's doing, or is just copying what someone else is doing  whether s/he's too clever (in taking advantage of undefined behaviour) or not clever enough by half (in just copying something that works without understanding that it's undefined behaviour).
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