When 1 or 2 people are building a project, making decisions is easy. You consider the options, you pick a strategy and you're the one to blame if it fails. If that happens, you pick up the pieces and try to pivot, but you're still in charge.
When you introduce more people to the process of decision making (even on low engineering levels), it creates a massive political overhead. People compete for visibility, so they will propose ideas even if they don't make much sense strategically. People have feelings, so they will often support ideas of their friends and dismiss the ideas of their enemies regardless of the business impact. People don't want to be responsible for failure, so they will often create lots of unnecessary low risk/low impact work instead of taking risks. So there will be indeed a lot of extra work, but in many cases the customers won't notice the difference.
The upside of this is fault tolerance - if one of the 2 co-founders leaves a company, it is doomed. If the same work is done by a team of 13, you can fire and easily replace 1-4 of them without much impact and investors generally like that.
Besides, you can actually mitigate the cock-up risks without going crazy on your head count: have focus groups, beta testers, roll out updates gradually, have a clear separation of layers and a mechanism to roll back a breaking change, etc.
It's one thing to build a WhatsApp-type app, focusing 100% of your energy on the core messaging functionality itself, and little else.
It's a completely different thing to build a revenue-generating business that needs to connect with an entire ecosystem of advertisers, ad servers, video servers, programmatic ad exchanges, analytics, brand safety, reporting, billing systems, and a lot more.
And we're not even talking about services. There's another big layer of complexity to keep those services running 24x7 (well beyond just SREs), ensuring ads are running properly, campaigns are fully optimized, advertisers are happy with what they're getting, etc.
Of course ideally the question would be “why not just have users pay you directly instead of relentlessly monetizing every bit of personal information you can squeeze out of them” but as an industry unfortunately it doesn’t seem like we’re there yet.
My interpretation of the GP's comment is that the advertising platform is not the core business but rather a secondary concern. Focus on your core business and outsource the rest.
Building an application is easy. Maintaining it, adding features, scaling it, integrating with other services, building platforms… those activities all take a lot of other work.
I work at a small company whose core product was developed primarily by one or two developers. We now have substantially the same functionality with an engineering team of ~13, but constantly have a huge amount of work we want to do. Things like scaling, refactoring legacy code, paying down technical debt, handling complex and tricky edge cases that could previously be glossed over, and building administration tools – these take up _phenomenal_ amounts of time, especially if you want to work at a sustainable pace. I can totally believe that larger software houses need a surprising number of engineers.