When talking about spectral sensitivity, we should forget the concepts of "primary" and "secondary" colors or color mixing. They are relevant when talking about human vision or color products, but when talking about pure sensitivity, the only thing that matters is wavelength - how the emulsion responses to different wavelengths.

To make this clear, I say "yellow wavelength" instead of just "yellow".

"Yellow" can be many different things; it can be a mixture of two distinct spikes of red and green wavelengths, or it can be a broad, continuous range of wavelengths from green to red, or it can be a single spike between the red and green wavelengths - this is "yellow wavelength". The point is, the last case (monochromatic yellow) does not record on BW film that is sensitized from blue to green, but the two former record on the very same film; and still, we see all of these yellows EXACTLY the same with our eyes. Color films, or digital cameras, would also render them same, as they are designed to somewhat match human vision by having overlapping spectral responses in emulsions. But, once we are talking about BW emulsion's spectral characteristics, we can forget all that. It's just the wavelength content that counts.