Hiring an engineer has a significant opportunity cost.
You're right that in the short term you aim to hire where the return is highest, but in the long term that means they're right that you'd want to keep hiring as long as you get a positive return.
You want to spend it on whatever has the highest return, which will frequently be on an expense other than hiring, and when it is on hiring, it will frequently be on hiring roles other than engineers.
There isn't really an opportunity cost to hiring more people when money is no object. More likely to be the opposite.
Why do you think they increased their headcount so much in the last few years? Simply because they could.
The fact there is finite budget and headcount doesn't necessarily mean anything. If you're limited by the number of quality employees you can recruit and onboard, budget and headcount are effectively meaningless because they're not limited by money.
How to apply it wisely is a different, much wider topic.