That will require investment in the construction of additional capacity to create that fuel
That investment won't come from tax, and won't come from airlines. Again this tax doesn't drive investment, it's not the point of it. The point is a flexible purchase agreement rather than regulation.
For your car there was a regulatory approach. The government said from day X you won't have more than Y amount of sulfur in the fuel. That fuel was already available so they can do that. SAF as you rightly pointed out does not have a lot of capacity (in fact the two largest SAF plants in the world had construction cancelled this year), so a regulatory approach can't be done. Instead they collect tax money and purchase what is available.
This is completely and fundamentally different from a carbon tax approach which looks at applying direct cost to polluters to get them to change their behaviour. At this point there is zero behavioural change expected by airlines and airports. The only behavioural change is expected on the consumer. This tax doesn't have a sunset date or a natural phase out through greening like a carbon tax does. It's not an avoidable tax through investment.