High gear clutch slippage

[Stewart Way]

Coming back from a test session a couple of years ago, a very experienced person commented that he couldnt understand why a 4 speed car would go thru the first 3 gears without clutch slippage and then slip in 4th. I said it was because the car makes more power in 4th than in the first 3 gears. He allowed as how that was the stupidest thing he had heard, the engine doesn't know what gear its.
My feeling is like on a dyno. The slower you bring the engine up at WOT the more power it makes. An engine at 1000rpm/sec test rate will not have as high a HP number as an engine tested at 600 or 300 rpm/sec. I have never seen it done but I would think the highest HP would ne seen at a steady state test. So in 4th gear the engine sees its slowest accel rate due to various resistances and therefore makes the most power.
True or false?
I bring this up because he make the same statement yesterday and I just let it go.