Depends how it’s set up. And even if your statement were true, a 150mm fork dropping 40mm is still less likely to tip you over the bars than a 120mm one dropping 20. Not to mention the effect of the slacker HA.
Using this example is perfect. Suppose your situation above, 120mm has gone through 20mm of travel, your 150mm has gone through 40mm of travel, you yourself have stated that there is a bigger change with a longer travel fork. With a larger change in travel on an impact, a larger shift in body weight is needed to counteract this shift that is tending to throw your weight forward.
Then you have the HA aspect, for arguments sake the 150mm sits at 67* static, the same frame will sit about 68* static with a 120mm fork. Using your scenerio, 150mm compresses 40mm, the resulting HA will be about 69*. Again with the 120mm fork, it compresses 20mm, the resultant HA will be about 69*, so dynamically the HAs will be the same, but the longer travel fork will have more tendency to compress making the sweet spot of balance shift further and more quickly.
Hopefully that’s making sense?
Longer travel forks will give more traction, it’s a balancing act between traction and handling (comfort too).