Probably meaningless. If Comcast is un-throttling speedtest.net there's no reason why they'd need to un-throttle your entire connection just because you're talk

Probably meaningless. If Comcast is un-throttling speedtest.net there's no reason why they'd need to un-throttle your entire connection just because you're talking to speedtest.net, since they can just un-throttle any connection talking to speedtest.net's IPs. More generally, if your response to suspected bad network is to open speedtest.net, and given that a lot of bad network issues are transient, it's not at all surprising that you'll have cases where you suspect throttling, open speedtest.net, and the connection problem just happens to go away at that point. So this just becomes confirmation bias.