It’s true that they use the correct JScript + rendering engines to each browser segment that is triggered. Along with asking the product authors I decompiled the app to see how the runtimes were being initiated (through an event spy product) and I can say for sure that it uses the correct files (all with matching checksums to those provided with the browsers). The product is a sound tool… and I’m saying that as a former full-time software developer. 
I was actually referring to the word “only” in your message 