This seems to be a lot of confirmation bias and use of data that is likely not significant.

For instance:

"The results were stark: Teams with more than one founder outperformed solo founders by a whopping 163% and solo founders' seed valuations were 25% less than teams with more than one founder."

How many of the 300 investments in their portfolio were solo founders? 10?

Solo founders are rare, and it's often harder to raise money as a solo founder. That means less companies have solo founders to begin with.

Source: I am a solo founder