That's incorrect. The old comcast system took congestion into account. I routinely ran 500-600GB/month offsite backups, but I scheduled them for the middle of the night and midday, and I never got a nastygram for exceeding the cap. The new system pretends that all bits are equally expensive, regardless of whether the system is currently slammed or twiddling its thumbs, which is a profit-maximizing lie.
I suspect it was selectively enforced. I had a similar usage pattern and did get a notice.