Told by field engineer can't use cable card + Tivo with 2gb up/down internet

Yes - the subject line makes no sense, but that's what I was told. Recently wanted to upgrade internet from 1gb down / 40 up to 2gb up/down. Also modified my cable package to the Ultimate X1 package. Made the changes online and the order acknowledged I was keeping my cable card and didn't raise any red flags. When the tech came out to install the modem he said I couldn't use the cable card with the new service and would need to get an Xfinity box. Made zero sense. How would switching to an Xfinity cable modem (from my own) impact cable tv service? Declined to do so and he packed up and left, saying the order would stay open on my account but should drop off in 30 days.

Fast forward two weeks later and I get a call from Xfinity saying there was an open order, and since my existing cable package was no longer available the system had removed it. (Great, super awesome.) Over the phone, he upgraded my cable package to X1 Ultimate without any issues with my cable card and we kept my internet at 1gb/40mb.

So it would seem there actually wasn't an issue. Or... was there? I'm so confused. Why would upgrading internet impact the cable w/ cable card side of things?