Why is مَذْبُوبٌ not geminated like ذَابٌّ?

Doubled Verbs: Complete Conjugation of ذَبَّ يَذُبُّ, Explanations and Essential Rules

• A long wāw separates the two bāʾs
• It has only one root bāʾ
• It is non-Arabic
• First radical is deleted