All you need to do is goto Apple webpage and read for yourself.
https://support.apple.com/en-us/HT205296
"When Wi-Fi Assist is activated, you'll see the cellular data icon in the status bar on your device."
Or you can intentionally cripple your wifi and watch it not work even if you have good signal. (I get about 10mbs LTE off wifi)
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Plus, aren't you the one that is having problems making it work the way you claim it works?
Seems odd to me, I am curious though link WWDC with a time stamp.