I would expect their salaries to be in the same ballpark as a senior engineer or line manager.
There shouldn't be any surprise that these are highly compensated positions.
They don't need to be masters at any one of those things (aside from organization and management), but they need to be well above average at all of them, which is rare nexus of talents to find.
Also, experience and skills like that are usually earned through entry-level work in other fields. Most of the good PMs I work with came up through other career paths (like non-profits, academia, print or tv production, paralegals, etc.) that are typically not so well compensated. They make lateral moves into product management for financial reasons, and they top out quickly. The highest paid PMs I know were mostly just former engineers themselves.
And the numbers in the article appear to be national, not limited to SV (despite the headline)...
"...31,146 interview requests from 1,848 companies made through Hired’s platform..."
"...Analyses of H1B work visas and Glassdoor data from Google, Facebook..."
Sales, product development, customer and market research, internal management, marketing - these are all critical skills for startups. People leaving to start their own businesses would shrink the pool, and put upwards pressure on wages.
I've also seen a lot of PM job ads asking for people with startup experience.
There were plenty of generic product manager jobs in other industries, but they were all wondering why she's worth so much if she doesn't have experience in their industry. Shifting industries, even with the same generic skill set, would have resulted in a 10-20% salary cut.