It's more likely that something like Cyrus or Perl will be a key part of the business at Fastmail than at some random similarly sized shop where email is someth

It's more likely that something like Cyrus or Perl will be a key part of the business at Fastmail than at some random similarly sized shop where email is something they use rather than a key part of the business, and hence worth spending time/money on. And there's economies of scale. A few cents from the payment by each of our customers adds up to a decent amount of money to spend on an open source project.