No, that is not true. Many have, but certainly not 100% of the developers. If you take a look at this thread, you would see that many bitcoin developers use IRC and mailing lists to communicate with each other. GitHub is also high on the agenda for them.

When it comes to altcoin projects, Bitcointalk is a great place to advertise those especially if the company has plans to run ICO and similar advertising campaigns. But I have come across news headlines mentioning various projects who have never done any advertising on Bitcointalk. For that, they can use Reddit, YouTube, Twitter, Discord, and other social media.