1.) Pairs or more of people working on a task may declare themselves to be an organization and call themselves founders earlier than an individual. An individual may be more likely to wait until they have a customer, are near profitability, etc to make that declaration.
2.) The summary says that "investors rarely, if ever, fund startups founded by a solo entrepreneur".
So the observations are confounded with solo founders overcoming this prejudice. This quote also points to a survivor bias in play. IE, solo founders must have higher performance than teams before getting funded, and less of the weak solo founders survive to be observed by the study.
I wouldn't be surprised if the article had a correct conclusion. Though it would be very hard to convincingly show causality short of convincing entrepreneurs to take part in a decades long experiment where they are randomly divided into solo founders and teams (which would be nearly impossible to get a representative sample of founders to agree to).