An article in Reuters describing where the 5% number came from:

Twitter Inc (TWTR.N) estimated in a filing on Monday that false or spam accounts represented fewer than 5% of its monetizable daily active users during the first quarter.

The social media company had 229 million users who were served advertising in the first quarter.

Twitter estimates spam, fake accounts comprise less than 5% of users -filing <-- An archive of the article, which can be easier to read