This also points to why free software and open source aren't the same thing.
Free software makes no promise to fewer bugs or other development methodology claims. Free software argues that the inequity of software non-freedom is unethical; software non-freedom is no way to treat computer users. Software non-freedom works against users spirit of cooperation and community. It is precisely this focus on ethics that the open source developmental methodology was founded to run away from.
There are still show-stopper problems with the article's ridiculous claim: by definition nobody knows all that happens in so-called "closed source" software, better known as proprietary software. Any claims as to proprietary software security are unverifiable even if one is deeply familiar with a particular popular implementation of some proprietary software. So the title ("Open-source crypto is no better than closed-source crypto") immediately makes a claim it can't defend.
The article also claims, without evidence, that "Paid security audits help but won’t find all the bugs, as they tend to be broader than they are deep.". That's quite a claim made on behalf of "paid security audits" -- they're "broader than they are deep". I'm not entirely sure what that means, but there's nothing in the article to indicate where one can find that survey data needed to back up that claim.
Ultimately the article poses no challenge to a free software ethics-based understanding of proprietary software: no matter how difficult cryptography software is to fully understand, no matter how insecure current FLOSS cryptographic implementations are, that's no reason for equating software freedom with software non-freedom. Cryptographic software is no exception to the need for respecting computer user's software freedom. Software freedom lets us do as we wish with our copies of the published free software and thus control our computers and treat each other ethically. Software non-freedom invites dependency and apparently incentivizes malware (see https://www.gnu.org/proprietary/ for around 400 examples of proprietary malware). As that page says, the "initial injustice of proprietary software often leads to further injustices: malicious functionalities.".
Why? https://www.gnu.org/proprietary/ explains:
"Power corrupts; the proprietary program's developer is tempted to design the program to mistreat its users. (Software whose functioning mistreats the user is called malware.) Of course, the developer usually does not do this out of malice, but rather to profit more at the users' expense. That does not make it any less nasty or more legitimate.
Yielding to that temptation has become ever more frequent; nowadays it is standard practice. Modern proprietary software is typically a way to be had."