Peter,
Hmm. I just reviewed the material found in https://en.wikipedia.org/wiki/Exponential_smoothing.
I will be the first to admit that I do not have an advanced degree in statistics, but I don't see any problems with the formula contained in SAP online help.
Anyhow, if you believe that a formula contained in SAP online help is incomplete/incorrect, you should raise an incident with them. Knowledge Base & Incidents | SAP Support Portal
Please keep us posted of the answers you receive from SAP Support.
Best Regards,
DB49