Actually you can get a full refill. I have refiled smaller 1 - 3 gallon, tanks off of a 20 Gal. tank many times; the trick is that the "donor" tank has to be upside down so you are getting liquid propane, not the gas on the top of the liquid, and the receiving tank has to be right side up so the relief overflow valve is doing it's job properly (and preventing an overfill), and that adapter won't allow that to be properly done.
I had one that DID work for small 16 oz tanks, (I had the refillable type (I didn't do the throw away types...), but it had a 10" or so side tube, and the bit that goes on the receiving tank was 90 deg to that so the receiver tank could be set upright.
It was intended to be used with a 20 lb portable and set the portable up upside down on its "handle/valve guard, then setup the tank to be filled.
The only way that adapter can be used effectively is on the end of a hose, but that will waste near as much fuel as you get into the small tank. The hose just has too much volume to be efficient. you could add a valve at the end of the hose and do a bunch to minimize the waste, but a small short feeder tube is better.