It has been a little while, but from memory the dot ball percentage for a lot of premium bowlers is up towards/around 40%, especially for the quicks. [I think spinners are more likely to concede multiple singles in an over, but perhaps fewer boundaries.]
If I adjust your calcs to get up to Siddle’s last SC average, I think it implies c. 38% dot balls, which seems consistent.
At times I’ve struggled to reconcile how bowlers score as much as they do - most only generate c. 20-25 points from wickets each game, and don’t bat or necessarily take a lot of catches.
40% dot balls adds close to 10 ppg for those who bowl their full complement, and explains a lot of the apparent gap that I was struggling to understand.