This is the sentence you are referring to from this lesson:
Tan pronto como almorcéis, recoged la cocina.
As soon as you have lunch, tidy up the kitchen.
Tan pronto como... works very similary to "cuando" in Time subordinate clauses. We could replace "tan pronto como" by "cuando" and the meaning won't change.
Cuando almorcéis, recoged la cocina.
As soon as/when you have lunch, tidy up the kitchen.
We are using the indicative here because the action expressed after tan pronto como is something that will happen in the future. It hasn't happened yet.
I am sure you've come across our lessons about cuando + indicative or +subjunctive. But if you haven't, have a look:
For your information, we will have more lessons with other time subordinate clauses, including tan pronto como, soon.