After 10 years, yeah, you've established a large enough set of data that that's probably a safe assumption. What we're talking about here is an in season ERA that's much lower than previous performance would indicate. Things like an unsustainable HR/FB ratio, a really high strand rate of baserunners, and low K/BB ratio would be dead giveaways that the low ERA isn't sustainable. Those things are factored into FIP and xFIP to give you a more accurate assessment of performance.