I just came across the mobile emulator
https://mobiletest.me/
It looks useful for checking if one's pages appear as intended on different devices.
As near as I can tell, that site is just sizing an iframe to match the size of the device and letting your browser render it normally in desktop mode wrapped in an image of a phone.
You can see, if you compare it to the mobile emulator in your browser, that it's the desktop experience (e.g. arrow cursor) not the emulated mobile experience (touch cursor).
So I *think* they are stretching the definition of "emulator".
You will probably get a better result, albeit without the graphic image of the phone, using the browser's built-in emulators. If nothing else, you will get a larger and more modern selection of devices (in Chrome if you edit the list, you can add devices all the way back to the iPhone 5).
Interesting marketing though. The first choice of example sites to render is the sales page for their agency!