You can add more agar or gently cook off some water. It would be easier to add more. That recipe is on the low side, 9 or more grams per liter is often recommended. If you get it a little too thick it will still work but is a little harder to get the protocorms out. Basically it is like cooking, you will just need to adjust the recipe.

I would really recommend getting an actual pressure cooker instead of using that part of the illustrated method. You should be getting the same consistency pre and post sterilization.