Speaking off the top of my head I would guess the reason is to ensure that you are contacting the water in full control of the a/c. If you were to attempt to contact the water in a stall and were a bit off in your judgement you run the risk of the a/c nose falling and the resulting attitude when contact was finally made would not be the most conducive to ensuring a successful evacuation. Basically as you have suggested.
As for POH’s, particularly those of GA aircraft, you must remember too who writes them. 50% is written by company lawyers, 25% by the sales staff which leaves only the remaining 25% to be written by the folks that know anything….the test pilots and engineers.
Regards