Are these people on a calorie surplus or deficit?
Depends what they want, but it doesn't have to be one or the other. There's a fairly broad area where you're eating enough calories not to be burning bodyfat, but not so much that you're adding it.
If a surplus then what use is Winstrol/Masteron when "lean bulking"?
Masteron is an anti-estrogen, reduces water bloat, is anabolic in its own right, can reduce PIP from other substances... winstrol I've never liked the sound of so never used it.
And surely a calorie surplus is always necessary when bulking even when using gear?
Absolutely not. See first point. Anyone who tells you you need to get fatter to grow muscle is an idiot. Probably a fat idiot.