Thanks guys, it makes total sense to me but I wanted some experienced corroboration.
I've been having a three-way dialogue between a big box retailer from whom I bought the tank and who seems very interested in my satisfaction, and the plumbing company who installed it. The plumber came back out with a rep from the retailer and I foolishly failed to question the above "evidence" when I had the chance, needing time to ponder on it. Foolish should be capitalized considering that the test that was run just prior was to bypass the water heater by running the cold into the hot line which seemed to restore flow. I would have thought that test would be pretty conclusive but apparently I'm easily distracted by shiny objects (pressure gauge).
If I run the cold line into the hot again, myself, (and this time take precise fill measurements) to to try to eliminate the water heater as the cause, are there any gotchas I should look out for before trying it? Looks straightforward enough (shut off water, make connection, turn on water) but I did notice the plumber grabbing the pressure release valve at some point. Total noob here.