Maybe. It does seem suspicious that those who upgraded using the Media Creation Tool are having problems. If it was a server issue I'd suspect everyone would be suffering similarly, but they aren't...
I'll just throw out that anyone upgrading from installation media (using the Media Creation Tool counts as that), has always required a new key to activate the upgraded Widows installation. For example, it was never possible to upgrade a W7 installation to W8 and then activate it with the W7 key. We were always required to type in the new key manually. The upgrade behaviour some of you are observing with the Media Creation Tool would be no different from any other version of Windows.
What is new and special about W10 is that W7 and W8.x license keys are converted. So far the only confirmed way that happens is with an in-place upgrade, but I've not seen anybody so far explicitly mention that the in-place upgrade had to occur online. That seems to have always been assumed however.
It will be interesting to see how this develops. Right now I suspect this approach will not have converted existing W7 and W8.x keys...