This is a potentially misleading statement.
If you were to provide the plants with good growth conditions,
(good lighting and possibly CO2 injection) then the tank would
be able to support more fish, as the O2 level would be higher,
and the water quality would be improved.
Without proper lighting, the plants impact would be minimal.
Plants use O2 all the time, day and night. But the amount they
use isn't a factor, even in a densely planted tank. If they
are growing well enough to remain densely planted, then they will
produce more O2 than they use.
-- Chuck Gadd http://www.csd.net/~cgadd/aqua