I think my math is right. I said
You can see the most likely outcome is 0 legendaries at ~40%.
Which aligns with your number (I rounded the 0.7%).
Where there might be a miscommunication is me saying
by getting the guarantee, we've increased our expectation by ~0.3 airdrops.
This is about a different metric. Instead of what is the probability of getting none, I'm now asking what is the expected value? The expected value of 299 is just under 1. The expected value of 300 is now ~1.3, a difference of 0.3. (expected value being the summation of the probability times the value across all possible values).