I repeated UhelligGudn's search and confirm the lack of a attack speed buff for Ball Lightning. Likewise, there is no code that is suggestive of it being implemented even partially. At this time, given the lack of any other supporting evidence for the existence of a attack speed buff, I would suggest that the tooltip is incorrect.
If the ability would be added it would make sense to have the buff be either 5%/5% or 5%/10% since that is the progression that is used for other attack speed buffs.
I also looked into the other language strings_db.lua files to attempt to confirm the tooltip, however I did not find updated versions of those files. Can anyone tell me what happens to the new strings in nonEnglish versions of Demigod when you update?