More affordable because the tax increases the holding cost of land which is factored into a buyer's purchase budget, much like how mortgage interest rates factor into a buyer's budget.
First let's assume the supply of land for sale is fixed. Of course land is in fixed quantity but I'm talking only about sales inventory – what is actively on the market to purchase. Say I can afford $4,000 per month. If property tax is $1,000 per month and interest rate is 6% maybe I say my purchase budget is $500,000. If interest rate decreases to 3% maybe I say my purchase budget is $700,000. Now consider if the property tax increased to $2,000 per month. Now if interest is 6% my purchase budget becomes $330,000 and if 3% then $475,000. In either case I still pay $4000 total per month. The economic incidence of the tax falls completely on the seller (the previous owner).
Now let's remove our prior assumption about the quantity of land for sale. we earlier established that land would be more available under a higher land value tax because of the increased holding cost incentivizing more sales. That increase in supply (of land for sale) should reduce the market price for land. I can either get the same land for less purchase price or more/higher quality land for the same.