That advice can get you in trouble on some days. The wind up high matters, but so does what it's doing right at the surface. A calm launch with a big shear at 300 is a recipe for a nasty surprise. Gotta read the whole column, not just pick one spot.
300 feet is a pretty specific number to throw out there. Shear can happen at way different altitudes depending on the day, I've seen it hit hard at 200 or even lower on a gusty afternoon. You're right that you gotta read the whole column, but saying 300 like it's some magic number might give someone the wrong idea. Really depends on the local setup more than any fixed height.