It’s essentially impossible to not swallow some of the bacteria in your mouth, and equally impossible to swallow all of them. And unless you’re getting dental cavities in your stomach, it doesn’t make any difference whether you swallow none, some, or all of them.
Don’t know where she got that idea, but I agree it makes no sense.
She’s apparently blissfully unaware that there are 10 times more bacterial cells than human cells in her body anyway. Spitting out the relatively small number of bacteria that grew on her teeth overnight makes no appreciable difference in the numbers of bacteria she is carrying. (Most of the bacteria she swallows will be killed and digested in the stomach anyway.)
We swallow bacteria all day, every day. The bacteria she’s referring to, though, aren’t harmful. They’re her own oral microfauna and, except for plaque and cavities, they’re mostly harmless. Although microfauna populations can shift, odds are that exactly the same kinds of bacteria will be reproducing overnight, every night.
Some bacteria can survive in the stomach, but most can’t. Helicobacter pylori is one, and it can cause stomach ulcers. Others have been found, but whether they can live there, or just survive while passing through isn’t known yet. It is known that the kinds of bacteria that thrive in the mouth don’t do well in the stomach. If you swallow those bacteria, they die, die, die.
What you have to look out for is bacteria in your food and bacteria and viruses on your hands. Microbes on your hands can be transferred to your eyes, nose, or mouth. If you want your mouth to feel fresh before put food in it - shrug. It’s not going to hurt you. If you’re trying to keep healthy, wash your hands a lot.
As a general rule, I find that it helps to start with the near irrebuttable presumption that anything any movie star says about health or diet is completely, irredeemably wrong at every level that it is possible to be wrong.