1 US teaspoon is about 14.8 ml

1 UK teaspoon is about 17.8 ml.

It goes on: 1 US gallon is about 0.83 imperial gallons

A teaspoon is 5 ml. I think you're thinking of tablespoons.

Teaspoons aren't particularly common in the US. If you do need to stir a beverage or add sugar, you typically use a tablespoon.

Seems like a weird statement. As someone who cooks and bakes, I use teaspoons all the time. Teaspoons for certain spices and seasonings are extremely common in recipes

Yeah, this isn't true. A teaspoon is a standard unit of measurement in US cooking.

Tableware sets in the US commonly come with at least two different sizes of spoon. These usually bear no relation to the "measuring spoons" used to measure volume, though they are handy for stirring tea or eating soup (for the small & large spoons, respectively).

For people who cook in the US, a set of "measuring spoons" is a very common kitchen tool.

"Teaspoon" and "Tablespoon" are units of measurement of volume, not bits of silverware. "Tea spoon" and "Table spoon" or "Soup spoon" are pieces of silverware, not units of measure!

The one you stir tea with is also probably smaller than a teaspoon (probably about half a teaspoon).

Yep, they're usually completely unrelated to the units of measure.