I hear it all the time, but, IMHO, I don't think that sailing a sunfish or any dingy is much like sailing a large (~30') keelboat.
You're right, it isn't. The purpose behind learning how to sail a dinghy first is to just get the fundamentals down, and
to make you sensitive to the forces the affect the boat.
When you take this sensitivity to a big keelboat, you tend to react faster, and sail better because you're more sensitive to what's happening.
People like me, who learned to sail a 25 footer first and then
sail a dinghy, find themselves flipped into the drink when sailing a dinghy, because they failed to react with the mainsheet or the tiller fast enough.