An average user having a few accounts doesn't make them bot/spam accounts. Those are used for active influencing/spamming.
This is from NY Post:
A former Twitter ad engineer told Jefferies in a May analyst report seen by The Post it believed the 5% figure is close to the real number.
“I would be very surprised to learn that the 5% number was meaningfully different and the company knew it was meaningfully different,” the anonymous engineer said. “In other words, I do not think the company has intentionally lied to the SEC. There are a bunch of very smart people working on this and my inclination is that they mostly got this right. Are there things that they may not have caught, and could someone point and say the number is really 8-10%? Sure. But that seems within a reasonable margin of error in my view. Could it be 25-50%? I really doubt it.”
The former Twitter engineer said Twitter does a multi-day sampling and looks for where logins are occurring, and if logins are a part of an IP address cluster. Twitter also looks to see if users are scrolling or liking too many tweets at an unreasonably fast speed.