Jumping a car doesn’t take a lot of energy. But it does take a lot of power (and therefore current). Small Li-ion car jumpers can be surprisingly capable.
A good idea to have just in case. :)
Insisting on changing 15k mAh to 15 Ah is the same as insisting on changing 15000 mAh to 15 Ah - which seems to be a matter of personal preference more than anything.
OP mentioned his mAh rating in 12 V but you’re comparing it to Ah ratings in 5 V. OP’s battery should be able to charge a full smartphone fine.
Also, most of those 10-20k battery packs won’t be able to jump start your car because they have greater capacity but cannot provide power quickly. There are high capacity battery packs that do jump start but they’re over $100 from the last time I looked.
Caveat - unlike a standard battery booster pack (lead acid), lithium battery does not take kindly to freezing temps, so it is useless to keep it in the car over winter. I wouldn’t expect to be able to start anything in the middle of winter up here unless it was kept indoors at room temp.
I use a lithium booster battery pack for an old car I use, and have used the battery pack to jump-start my car multiple times on a freezing-cold day (sub 10f) while trying to locate a store with a new car-battery in stock.