People imagine some special mechanism of transfer, but the reality is that we get bacteria from inside and outside of our bodies all over stuff, all the time, and we’re crawling with it. The babies would be introduced to GI tract flora pretty much immediately even if you tried to prevent it (unless you prevent it with antibiotics of course).

Exactly. Even just the bacteria on mom's skin will be transferred during breastfeeding.

Defecation is a common part of the birthing process.

I think there is even some evidence that cesarean births have higher risks of some infections because of not being exposed, I even recall that they did some things with babies being born with cesareans because of this.

But the feces is sterile. It's not even feces exactly, it's meconium and doesn't look like feces and doesn't smell at all (it's basically the remains of cells from the intestine and the amniotic fluid the baby swallows).

There are no bacteria in the womb.

Presumably he/she meant by the mother.

Oops. I read too fast.

Yes, the baby would be ingesting all sorts of stuff during the normal birthing process.

In a related note - families tend to have very similar biomes. Oral-fecal transmission happens despite the highest level of cleanliness.