I'm aware of why it makes power per displacement and why it burns more fuel than an equivalent four-stroke, but the fact still remains that it makes more power with less displacement, fewer parts, smaller physical size, and lighter overall weight.
More power with fewer parts actually does mean mechanical efficiency. The relationship between fuel consumption and power produced only speaks of fuel efficiency.
Acting or producing effectively with a minimum of waste, expense, or unnecessary effort.
able to function without waste: capable of achieving the desired result with the minimum use of resources, time, and effort
working productively with minimum wasted effort or expense.
None of these limit efficient to mean only fuel consumption. They all speak of productivity with minimal effort or waste. In the context of number of moving parts, physical size, weight, or displacement the two-stroke is still more efficient than the four-stroke no matter what you think the word means. In terms of fuel consumption the two-stroke is less efficient.