Ya... that's really the underlying issue. When the addon does a full rebuild of stats, it goes from whatever start date XenForo is telling it to start at, so if it's told to start at 1970, it will do that... and that's going to take an insane amount of time since Cloudflare stats are generated from API calls.
As a test, I triggered a full rebuild on one of my sites and captured the timestamp that it started at, and it's as expected (doesn't universally start at 0 (1970)). In my test, it started at a timestamp of, 1686441600 (which is June 10, 2023... which is what it was supposed to be). Even that is going to be a long time... 422 days x 11 API calls per day = 4,642 API calls that need to be done to gather the stats for that period of time.
Another issue is Cloudflare API limits, so even if easy API call could run instantly, there's still an API rate limit of 300 GraqhQL queries per rolling 5 minute window. So doing 11 queries per day, there's a rate limit that would cap the speed at 27 days worth of data per 5 minutes. So even one year worth of data would take a minimum of 68 minutes due to API rate limits.
It probably does make sense to simply ignore stat rebuild requests for Cloudflare data because it's never going to be fast, and it will also never actually work unless it's less than 27 total days the site has been online.