Tim+ expressed precisely :
Nonsense! Have you actually tried it? Ive syphoned water plenty of times
and never had to keep the outlet underwater.

As have I, it works fine on small pipes, but have you tried it on
larger pipes - I have. Air rushes up above the water from the discharge
pipe and soon breaks the syphon effect. If the discharge is kept below
water, then air cannot be sucked in.