The texel block size of each GPUTextureFormat is constant except for "stencil8", "depth24plus", and "depth24plus-stencil8".

But stencil8 has a constant texel block size (according to the table in Depth-stencil formats).

The stencil8 format may be implemented as either a real "stencil8", or "depth24stencil8", where the depth aspect is hidden and inaccessible.

Even if implemented as a depth24stencil8, the texel block size of the stencil aspect should be 1.

I guess the text refers to the whole texture not having a constant texel block size but it's not clear since depth32float-stencil8 is in the same boat but not mentioned.