Mary and Bob went to the store. They went to the store.
Q: Who are "they"? Who went to the store?
A: Mary and Bob went.
In this example. "they" are Mary and Bob. You may know their names, where they live, if they like fish, where they study, etc.
Sometimes, however, the English language uses "they" to mean other people in general, not specific other people (such as Mary and Bob). When "they" refers to non-specific people, it is called an "INDEFINITE PRONOUN".
Let's try some examples:
1-They say it is going to rain tomorrow.
Q-Who are "they"?
A-We don't know their names, but people in general have told us they think it will rain tomorrow.
2-I hear they are going to build a house on the hill.
Q-Who are they?
A-I haven't met the family, but various people in neighbor have said that there are plans to build a house on the hill. I don't recall when I was told that nor by whom, but it is something that I heard. Or did I read it in the newspaper?
Now let's look at your text:
They'll probably have lots of people swimming their last little way to Hong Kong.
Q-Who are they?
A-I have no idea. Maybe "they" are employees or owners of airlines. Perhaps they are government officials. I can't give you any personal information about them because I've never met them, and I don't even know their nationalities and jobs. "They" are just people somewhere.
Changing the subject slightly, the function of indefinite pronouns is found in many European languages, not just English,but the function is expressed without the use of "they". In Taiwan do you see signs that say "English spoken"? You probably do. Who speaks English? Someone in the store probably speaks English. Do we know his age, where she lives, where they eat dinner on Sundays, etc.? No, but they exist and are real people. A Brazilian speaks Portuguese, correct? If he wants his potential customers to know they can speak Portuguese in his business, he will use a sign that reads "Fala-se portuguÍs". The literal meaning is "Portuguese speaks itself." A free translation is "Portuguese is spoken here." A sign in Spanish would say "Se habla espaŮol". Again, the literal translation is "Spanish speaks itself" but a better translation is "Spanish is spoken here."
Have you heard, "You should see a doctor at least once a year"? Who is "you"? Is it Heidi? No, "you" is being used to mean "everyone". Do we know their names, if they have a cat, which books they like, etc.? No, we don't. In this example "you" is being used as an indefinite pronoun.
Your English is quite good. Keep up the hard work!
Student or Learner