The sentences have different meanings. The first one refers to a singular item, the second refers to multiple items.
Look at that pond. It's quite full up this year
Look at those ponds. They're quite full up this year.
It is possible that "they" is being used to refer to a singular person.
My cousin just ate a whole loaf of bread, so they are quite full up.
You don't use "it" to refer to a person (unless they specifically request it)
These aren't dummy pronouns. The dummy pronoun is usually (alway?) "it":
It's been fine this week.