I fully sympathize with the OP on this one. I looked at the website and and under the Basic option is written "Battery Included." Yes, it doesn't have a green tick next to it, but it is confusing. Why should you have to check with multiple sources to get accurate info? Surely the manufacturers website should be the de facto authority?!
The Festool UK site is much clearer. Under items included it states "without battery pack."
https://www.festool.co.uk/Products/Pages/Product-Detail.aspx?pid=561737&name=Cordless-plunge-cut-saw-TSC-55-REB-Li-Basic
I am sure Brent/ Festool will redesign the website to be better soon.