That isn't outside the realm of possibility. As Ciaopec stated, the process isn't as simple as it may seem. If the shop is repairing it properly, there is a fair amount of other pieces that they would have to remove before they can replace that quarter panel. Something that a lot of people don't take into account is the domino effect of removing pieces, as well. Sometimes it isn't just as simple as taking out the door or just the headliner (and this isn't specific to Tesla). You may need to disconnect safety restraint systems, remove glass, remove additional trim pieces to get to the trim pieces that were what you needed to remove originally.
Since you were hit on the left, the charge port and all associated systems would have to come out, as well. In addition, there is reprogramming that has to be done to the sensors in the rear bumper. We've also seen some wacky instances where after the car is torn down, there is secondary damage that is related to the loss but wasn't evident at first glance. We've had to fix autopilot issues when it didn't seem as though it would have anything to do with that. In hits like this, we've actually even seen suspension damage to the strut believe it or not. I'm not trying to make this a bigger deal than it seems to be. This is strictly based off of the hundreds of Teslas that we've seen roll through our multiple locations. After you properly tear a vehicle down, you would be surprised at the amount of damage that shows up.
Because they're a TAB shop, they're also probably treating it as we would - with extreme caution and care. Tesla audits for their TAB program are getting increasingly strict. The company has been trying to weed out the collision centers that don't perform and are moving towards a better customer experience in the entire repair process chain. Due to that, a lot of TAB's are making sure they get everything they can in the estimate. We don't want to miss anything and have it become a problem later on. Long story short, our estimators took a look at it and agreed that this is in the range of 10-14k just based off of a very rough visual estimate from the two photos you attached. This quote may be slightly high, but it is hard to tell just based off of two photos.
You could probably get it done cheaper at a non-TAB shop and they may do an alright job; however, I wouldn't recommend it. We've had Teslas come in from non-TAB shops that don't have Tesla support/training and they're a wreck. We end up repairing more than we would have had to. There was one instance where they welded panels that were supposed to be bonded/riveted. It wasn't great. We don't have any skin in the game. If you want, you can PM me a copy of the estimate you were given and our estimators can double check it to make sure the shop isn't doing something to fleece you.