This article addresses a subject that is hard to prove statistically and one that varies greatly from person to person. The main claim of the article though, is that people CAN be happier when single, but there isn't necessarily a cause and effect relationship between the two.
This article originates from Buzzfeed, which is a source that thrives off of sensational headlines and opinions. Often times while they can be interesting, Buzzfeed isn't a great source for trustworthy reporting. Focusing on specifically the author, she has written a book as well as hosted a podcast. She has experience with the topic as she has been single for a long time. This can be both good and bad for validity as she has firsthand experience with the topic and can speak from that experience, but it also leads to a lot of bias on the topic and possible exaggeration or withholding of information on the topic.
I was looking at other articles covering the same topic, and came across this one. The article is slightly different, as it focuses on the reverse claim, that single people are less happy than people in a relationship. The article sources research data showing that people who are single are unhappier. But the article also focuses on a more important claim: "those who dismissed relationships as unimportant were more satisfied with their lives than single young adults who were less dismissive of romantic relationships."
The ultimate point of this article is that the happiness of people based on relationship isn't necessarily a result of being in a relationship, but more the mindset and views of people regarding desire for a relationship. Ultimately, this is a really cloudy topic with no definite answer based purely on the happiness of anybody who is single. The true determining factor of relationship based happiness is more what you make of being in a relationship rather than whether or not you are in one.