The lid probably isn't airtight, therefore adding an airlock would be kind of pointless.
Check out the Cooper's DIY fermenter. It works on the same concept (no airlock). I've been brewing in mine for just over two year without any issues at all.
The lid just keeps stuff from falling into your beer. Positive pressure inside the fermenter from CO2 production will keep anything from floating up under the lid.
The garbage bag the guy mentioned was probably just to keep direct sunlight off the fermenter or help trap in heat if brewing in a cold area.
Personally, I ferment my beer for three weeks, bottle condition at room temperature for three weeks and then try to have them in the fridge at least three or four days before I drink them (ie. stick beer I plan to drink on the weekend in the fridge on Monday). That's a 6+ week turn around on a batch. If you drink a lot of beer it's in your best interest to establish a pipeline. I always have a batch of something in my fermenter. A lot of guys on here continuously have several batches going.