I'm not an expert on this but I think the point was market cap is the value of all outstanding shares. Tesla raised money through venture capital while GM raised money by going to the bank and getting a loan.

Think about buying a $1M rental property. One guy gets 9 friends together and they each put up $100k for 10% of the house. The house/business is worth $1m. The market cap is $1m.

Another guy wants to buy a $1m rental property but he goes to the bank and gets a $900k mortgage. The house is worth $1m but the market cap is only $100k.

Since tesla and GM raised money very differently their market caps are not comparable.