Lots of people get batteries from the car manufacturer. My views might be colored a bit because of my long history with BMWs, but BMW has used AGM batteries for quite a while, AND a battery change also required "re registering" the battery with the car with a specific process that required either a computer with an ODB to ethernet cable and specific BMW software, or later, a wireless ODB adapter and specific iPhone apps.
"Regular" BMW AGM batteries (for lowly 4 series) are like 500-600 installed at the dealer, but the battery itself is like 300 ish even if you go after market, and then spend the money on the items to register it yourself (did you look at the price for the BMW M4 Battery @ra88it listed? its about 2k.. yeah 2 thousand dollars). So yeah your not "cruising down to nappa auto" for your typical BMW AGM battery. No idea on the tesla, but I would likely go to tesla for it at least until the car is out of warranty.
I know tesla does not = BMW, but coming from that brand, the thought of an AGM battery does not surprise me, and 2-300 for a battery sounds actually cheap to me...
I'm distantly (in-laws) familiar with BMWs and 12V batteries. That's a, uh, unique thing they have going there. Always issues, always super expensive, always harder to get at than you'd expect (I remember needing trunk access for some reason once). I'm sure you'd glad to be a convert
I have a bit of a bubble to be fair. My dad's a mechanic. Many of my friends and coworkers can be classified as frugal. The thought of getting a 12V battery from the car manufacturer was almost never a consideration for those in my circle.