This argument has been used by ISPs for as long as I care to remember and it's always been BS. I used to work for one and I cringed every time I had to say that

This argument has been used by ISPs for as long as I care to remember and it's always been BS. I used to work for one and I cringed every time I had to say that line to a customer, but that was our official position even regarding speed test sites. ISPs know even most savvy customers wont be able to prove beyond a shadow of a doubt that they are being throttled/not receiving their promised bandwidth and so they use that grey area to say "Yeah, must just be that server or a router or something."