What he means here is that the belt won't directly just support your back; however, it can prevent back injury if your core is a limiting factor in order to maintain strict form (it often is when working with heavier/maximal weights, which is why competitive lifters warm-up beltless but eventually strap it up for their heavier sets).
Still, lifters can grow a dependency on the belt, especially if implementing it early in their lifting careers. If your low back feels vulnerable while deadlifting, it's more likely improper form/weakness somewhere. The belt will likely just mask a weakness that you need to fix. I recommend not using a belt at all until 3+ plates feels comfortable on your back with good form and 2x BW feels good off the floor beltless. If you can get to that point, you at least have a moderate foundation of core strength.
Another thing to consider is if you are trying to focus on building strength or bodybuilding. If you are bodybuilding then a belt can be a useful tool with lower weights because it allows you to focus on the muscles you are targeting when squatting/deadlifting while not having to be limited by core strength.