Is it true that Facebook limits your audience reach AFTER you've paid to promote or boost a post because they know you'll pay them again to promote?
Yes, it's true.
First Facebook shows that your post reaching to many people and can reach to bigger audience if you boost or promote your post. Once you pay for that then they limit it, and wait for you to pay more.
Audience limit is depends on your budget. Facebook select the audience based on your budget: