I will take a stab at this. I believe the only reason it would say that is that as I recall, schedule 80 is VERY thin walled and low pressure unlike schedule 40 which can take alot of pressure. Since you are (I hope) use this around your pool, what? 30psi probably no more than 40psi?
I my opinion, you should not have a problem with excess leaking or failure with the pressures we are talking about in regards to a swimming pool.
Bob E.