Its not a weak and inflammatory argument to call Windows Update a backdoor. If push comes to shove and its very profitable for MS (or they're threatened by the NSA, or any other "really good" reason), they can and very likely will automatically install something on your system that you don't want. Or replace the default search engine with Bing. Or uninstall something. Or make your machine unusable.
We've already seen mild examples of this (see U2's album)
SaaS (especially web-based) doesn't have that kind of power over your machine (or over other services you use)
Microsoft Windows doesn't have a backdoor to data on your android phone, but its backdoor is still universal within the scope of your your Windows installation'. Do you have a more appropriate term he could've used? I'm not familiar with one offhand, if you know one I'd appreciate you sharing it.
Microsoft's "universal backdoor" is Windows Update I'm pretty sure. And our telemetry is the spyware. And yes, you could likely attempt to alter a copy of Adobe Reader through Windows Update, but so could any installer under the sun. It's a weak, inflammatory argument to call updates a backdoor. Indeed, the argument against updates will boil down to "well it's not free so how do you know it's not doing x". Which becomes the argument that all nonfree software is/can be backdoored.