I think you may have read the data wrong, in two places it says

Most rental properties – about seven-in-ten – are owned by individuals, who typically own just one or two properties, according to 2018 census data.

and

Businesses own larger shares of units because individuals, while far more numerous, tend to own one or two properties at most, while businesses’ holdings are larger. In fact, 72.5% of single-unit rental properties are owned by individuals, while 69.5% of properties with 25 or more units are owned by for-profit businesses.

The first sounds like most (read: more than 50%) do, but, and I may be reading the census data wrong, it seems like less than 20% own a single property for rent. The use of typically would indicate that most do, but they don’t actually include the data in the article, which is odd and worrying.

The second also looks like it agrees with your assessment, but it actually kinda says the opposite- 72.5% of people who own single units for rental are individuals. This is surprising because it means there are 27.5% of single unit properties that are owned by businesses. However, it doesn’t mean that 72.5%, or even 50% of individuals (individual landlords) own a single unit to rent.

This article all comes from the 2018 census, when the 2021 census is also available, but I wasn’t able parse either very well.