Matrix Expression Comparison Operator

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The docs state (emphasis mine):

Comparison: A cmpop B, A cmpop alpha, alpha cmpop A, where cmpop is one of : >, >=, ==, !=, <=, <. The result of comparison is an 8-bit single channel mask whose elements are set to 255 (if the particular element or pair of elements satisfy the condition) or 0.

However, if img is CV_8UC3 and I write this:

cv::Mat res = img != 0; 
cout << res.channels() << endl;

This prints: 3

Based on the docs it seems that it should have printed 1.
Is this a bug in the docs or in the library?