Isn't this exactly what the author is talking about.
That the VC industry biases against solo founders and so they end up with less term sheets during a round and thus lower valuation and lower success rate.
The take away, even assuming that "on paper" solo founding is better, if you want to start a VC-backed company (or a company which will eventually raise VC money) this really isn't the topic to get quixotic about and you should get at least one co-founder so you can improve your chances.
A VC-backed company, as opposed to a kickstarter project, is a company which starts from zero and then is worth a billion dollars in five years. That involves exponential growth and stress which, unless you've attempted it, you can't even imagine.
Potential solo founders: knock yourselves out, but when you've gotten your 100th "no" and your cash reserves are running on fumes, this author and their flawed research paper they are citing out of context will be of little comfort.
I do agree that solo founders should probably find a co-founder in order to raise but that has nothing to do with potential success and everything to do with optimising for what VCs expect.