Repeat that today and you'll most likely end up with a Google Penalty for guest post spam.
What really worked for Buffer was that they found a way to attract a lot of audience and share their content. They did this by writing a lot of life hack posts. That strategy, obviously, has been milked. If you want to write self-help, Medium.com is likely the place to be.
I agree with the quality over quantity part.
However, quality content is just the entry point today. Networks are making it harder for you to distribute the content unless you pay them money.
My experience is that unless you create some eye popping content that can go viral or be pickled up by media outlets, it's better to stop doing guest posting and focus on your blog. Write great content and spend 5X the time promoting the content.