The flux definitely wasn't to blame in the first attempt
Correct, it wasn't. It was me, I thought I saw the chip self-center (indicating proper solder attachment) but I was mistaken, hence I removed the heat too early.
, expecting all of the solder to wick through the vias is asking for trouble IMO.
No, it's not. If there is enough surface tension under the chip from the solder to self-center the chip, then it's gotten through properly and will be a good connection.
You would have been better off using a bit of paste or applying a small amount of solder to the top side of the PCB.
The whole point of the video was to show how to do it without flux (not everyone keeps flux in their fridge), and if you are relying on the thermal conductivity through the die and up through the bond wires and down the pins and then onto the pads to get a good reflow solder connection on your pads, then you are doing it wrong. Hence my two-step technique of thermal pad first, then traditional drag soldering.
You can argue about whether to tin the pad first until the cows come home. Personal preference. I showed that extra step is not required.
Dave.