He didn't say nulls were treatable with EQ. He said toughs might be, and I agree with him.
Trough is a point in the cycle where minimum amplitude exists. In another way, it is a point on the negative side of a wave where minimum amplitude exists.
minimum amplitude is not a slight dip in frequency response, it is a deep dip. It is not wise to try and fill in deep dips in a frequency response of a speaker/room interaction. It is better to move the speaker, the MLP, or use bass traps or acoustical panels to eliminate the reflection pattern that creates it. Using EQ for this purpose is not wise, and can create more problems than it solves.
If you are doing room EQ, it's possible that it's somewhat or completely fixable with EQ depending on your situation, so it's worth trying.
You can try, but filling in dips (depending on how deep they are) is not wise as it puts more stress on the amps and the drivers themselves. We know that for a fact, so it may not even be wise to try.
It's possible Amir, myself, and Matias are all using that word incorrectly though. If not, then you are.
It is also possible nobody is using the word incorrectly, but differently. When I think of minimum amplitude, I don't think of a slight dip in frequency, I think of a deep one. I would not touch a 5db dip with EQ, especially if it is a high Q dip.
If this is the example you are pointing to
The deepest dips in this example are what I would call troughs. The five deepest dips I would not touch with EQ but would pull the peaks down instead. Even the lesser dips I would not touch with EQ, as that would be about 5db of boost for each. This is where absorption would be more effective than EQ.
Consider a xover that is not quite right and leaves a substantial phase difference between drivers. This can result in a trough or even a null if out of phase by pi. It’s also easily correctable.
The only way you could correct this is not with EQ, but by making a new crossover. A null cannot be corrected unless you move out of it.