I was at an aerospace company. In aerospace, specs matter, because part A has to plug into part B. You can remove the Pratt and Whitney engines from an airliner and substitute Rolls-Royce engines. One is not emulating the other; they both meet the same spec. DoD used to be big on having multiple sources, all making interchangeable units to the same spec.
We used to say, "if A won't work with B, check the spec. If A doesn't match the spec, A is broken. If B doesn't match the spec, B is broken. If you can't tell, the spec is broken.
Much of this worked in the early TCP/IP days because DoD was funding most of the players, both industrial and academic. They wanted interoperabilty.
There's much less of that today. Compatibility tends to involve reverse engineering the dominant player's product.