Automattic does provide monitors for all employees. In fact, they will arrange a session with an ergonomics consultant upon hiring you to set up a proper home office, as well as give you an allowance to buy a proper desk, chair, keyboard, etc.

At the San Francisco office, there's a table with cinema display monitors you can plug your laptop to, they're just out of frame on this picture.

I'm 100% sure the people on this photo are not using the external monitors because they don't want to. Some people prefer a consistent workflow everywhere (be it at home, the office, Starbucks or at the beach) and you can't quite get that with an external monitor.

Source: Worked at Automattic