I agree with you that a person that uses this website does not really 'understand' matrix multiplication in any deep way, though they will have slightly higher chance of understanding the algorithm at face-level.

I don't think that makes the site worthless or worthy of the some of the scorn I'm seeing in here.

I strongly agree with the complaint that "this completely misses the point" is super negative and dismissive and should be condemned.

If we-the-community want people to keep making things for fun or to try to help people, we have to criticize them graciously instead of saying that they are so wrong as to be useless. Not because it is moral or proper to do so, but because in practice dismissive and scornful negative feedback makes creators less likely to create new things or to try to improve old ones, and that's bad.