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A very clever backdoor indeed. Good plausible deniability too, since this is such a common typo for C programmers, and one that isn’t even caught by syntax checkers, since it is still perfectly valid syntax.

Yes, it's actually a C idiom. E.g., to process the contents of a file:

while (bytes_read = read(buffer)) {
    // Work with buffer
}
// ... Dropped out of read loop because zero bytes were read

Commonly used compilers can be set to warn when the above is used, requiring it to be changed to:

while ((bytes_read = read(buffer)) != 0) {
    // Work with buffer
}
// ... Dropped out of read loop because zero bytes were read

to avoid the warning. Of course, the assignment still takes place whether the target is bytes_read or current->uid.