The phrase "emphasize that not" is grammatically correct and can be used in written English.
It is typically used to clarify or stress the negative aspect of a statement. Example: The professor emphasized that not all students would pass the exam, even with extra study time.
Some Crime Stoppers coordinators say their program appeals to community spirit and emphasize that not everyone who calls is after money.
As the head of the British Child Exploitation and Online Protection Agency, Peter Davies, put it: "It's important to emphasize that not all members committed a crime.
The authors emphasize that not all systems have these weaknesses, although many do.
It is important to emphasize that not all QSAR models are developed for being used in routine prediction and therefore not all of them are suitable for being used in production environments.
In this paper we review the reactivity of oxygen on a well-defined copper surface to emphasize that not all sites on a surface are active sites, and that oxygen in different environments, even on such a well-defined material, has very different reactivities.
It is important to emphasize that not all disasters, particularly natural disasters, can be prevented, but the risk of loss of life, injury, and damage to property can be mitigated with good evacuation plans, land-use planning, environmental management, and building codes or standards (Wisner et al. 2004; DFID 2006; UNISDR 2013).
Now, I want to emphasize that's not easy.
It is important to emphasize that ∠ is not a rigorous analysis; correlation among parameters is only verified in parameters pars.
But he did not emphasize that point.
The soldiers, sailors and marines emphasized that not all troops took part in indiscriminate killings.
The city emphasized that not only would it not finance any part of the program, but it also expected the winning bidder to share its profits.
