A TSB is not a warranty nor is it a recall, it is merely a factory recommendation of how to repair a known issue.
So:
If the repair is covered by your powertrain warranty, all costs, parts and labor, are covered and there is no charge to you.
If the repair is not covered by the warranty, all the costs, parts and labor, are paid for by you.
Personal opinion: The impact damage is not sufficient to have caused the split in the end tank, and the repair should be covered assuming you are still within the terms of the powertrain warranty. However, my personal opinion has no weight on your particular situation, the repair has already been done, and you are probably stuck with whatever decision the service manager has made.
Assuming your powertrain warranty is still in force, you could try to get GM customer service involved. The fact that the intercooler was not repaired along with the rest of the impact damage works in your favor, in other words, it wasn't damaged enough to require repair, therefore, the split in the end tank should be covered by warranty.