Hello, thank you for useful lectures.
I'm reviewing all the lectures for final exam, eventaully.
And I have a grammatic question.

직장인이라면 여러가지 이유로 다니던 회사를 그만두는 건 어떨까 상상해 본적 있으실 것 같은데요. 그런데 이렇게 가끔 상상해 보는게 아니라 10년동안 퇴사를 준비해왔고 지금은 아주 만족스러운 시간을 보낸다는 분이 있습니다

There may be employees who have thought of quitting their job for many reasons.However, instead of just thinking about it there is someone who planned it for ten years and is now fully enjoying their life.

The bolded part is my question part.
Is someone be replaced by they or their in a context?

I saw the same case in another lecture of your lectures in this program, too.
I thought someone is he or she so, it should be singular but on the other hand he or she is they, I guess.

How should I understand someone and their in this context?
Thank you for your kind answer all the time.