Isn't matrix multiplication already a convolution? You are rotating the right hand side matrix anti clockwise 90 degrees and then convolving it upon the LHS matrix from top to bottom.
discrete convolution is cₙ = Σᵢaᵢbₙ₋ᵢ
there is no way in which the indexes into the input matrices in a matrix multiplication are formed from sums or differences of indices and dummy variables
however, convolution is a matrix multiplication, specifically multiplication by the circulant matrix of the convolution kernel
hth, hand
i typed special characters with the compose key; cf. https://github.com/kragen/xcompose
not as easy as latex but more compatible