Cartridges in this class, you expect a trade off. There might be a few 222Remingtons out there that have maintained acceptable accuracy since the 50's, but there is no free lunch, shooting the big boomers. One I am familiar with is 6/284, (same thing, only different) and which could be considered "over bore" for some people. If you are not careful, you can lose accuracy while developing a load. In this particular example, less than 600 rounds. Other people, sensible loads and a slow twist, they might squeeze twice as many rounds, but then, why not just get a 243?
Without knowing anything about who said what and what their frame of reference might be; I would be very skeptical of claims of 5,000 accurate rounds from a 30-378 regardless of the propellant used. Put another way; Not in my world. BB