The phrases "in early 2021" and "by early 2021" are similar, but they have slightly different meanings. In early 2021 means "during the first few months of 2021." It implies that the event or situation happened sometime within that time period. By early 2021 means "no later than the first few months of 2021." It implies that the event or situation happened by the end of that time period. So, in the sentence "By early 2021, he was living in a tent, under a tree on the edge of a highway in Portland," the phrase "by early 2021" implies that the man was living in a tent by the end of the first few months of 2021. Here is an analogy to understand the difference between the two phrases: In early 2021 is like saying that you ate breakfast "in the morning." It could have been any time in the morning, but it definitely happened before lunchtime. By early 2021 is like saying that you ate breakfast "by the time the sun came up." It means that you had to eat breakfast before a certain time, but it doesn't say exactly when you ate breakfast.

In this context, if you said 'in early 2021', it would imply that he is no longer living in a tent, but he may still be with 'by'. Also, in this type of narrative, 'by' is conveying a culmination of a series of previously described events, and is a somewhat literary way to use the word.

"In early 2021" is simply stating a fact. At that time, he was living there. "By early 2021" means "no later than." It also carries the idea of a continuing process, with a point being reached no later than a particular time. "We will be in Vermont in 2024." "When will you be there?" "We don't have firm plans, but we will be there by April 8th at the latest, because we hope to see the solar eclipse." Your article--which I found by means of a web search--describes the sad story of a man whose problems became worse and worse until he became homeless. Later in the article says "When Anthony Saldana arrived in the city in 2018, he first rented his own place and then bunked with his sister and her partner, Phillip.... As the pandemic wore into 2021, Mr. Saldana left his sister’s house and started sleeping outside." So it is perfectly correct to say that in early 2021 he was living in a tent. But it would also be correct to say "His problems grew worse and worse. In 2018, he was living in his own apartment. By [year?] he was living with his sister. But by 2021 he was living in a tent."