That is correct. There is a mindset shift that has to take place, you have to see ethereum as the currency your trying to boost versus trying to boost the USD value, irrespective of ETH's fluctuations. Otherwise, in your example, you would simply not have bought any NEO and had more USD just from hodling ETH.

Additionally in your example, If ETH price significantly drops, your buy order might go through despite NEO price not changing. But that should not matter if you still have faith in ETH and believe its USD will bounce back.