Food manufacturers and the FDA have tightened the rules up in recent years, but yes, labels are estimates, but usually “close enough” estimates. True with a packaged food and even true with things like meat and eggs. That’s why I use terms like “about 300 calories, give or take 100-ish” and allow for tweaking based on results – weekly weigh-ins or a 3-point tape measurement around the belly area.
Natural variances in activity level are an adjacent issue here, too. Is someone really in a 300-500 calorie surplus on Sunday if they play a spontaneous game of pick-up basketball for two hours? Gets tricky.
But having a calorie count in mind is a good place to start, especially for someone who’s never done it before or has a tendency to over-diet or over-bulk. They need some sort of compliance factor, at least for a while.
Generally, I don’t count anything these days. I’d rather keeping a few principles in mind (no added sugars, no junk fats, high protein, etc.). But with a specific strategy like this, there’s value in counting until you get the hang of things.