See How Competitive Matchmaking and Rating Works (Season 19), especially the summary and the references for what Blizzard has said on the topic.
You’re missing a lot of info here. How many games are you comparing on the two accounts? What is the SR of your alt? Lower SR games are expected to be lower quality, but also there are so many random factors that it is difficult to compare game quality between two accounts without a lot of games.
That post is gibberish, and contradicts pretty much everything the developers have said about how matchmaking works.
Level is not taken into account in matchmaking. If two accounts are played by the same person, the same way, for 150+ games they will end about around the same rank.