Their explanation is incorrect, but their conclusion is correct.
You are only exempt from Work Related Activity if you are in the Support Group, if you receive Carer's Allowance / Carer's Premium or if you are the lone parent of a child under 5 (
Regulation 3 of The Employment and Support Allowance (Work-Related Activity) Regulations 2011 (SI 2011/1349)). Unless you fall into any of these mandatory exemptions, you can only defeat a mandatory requirement to undertake Work Related Activity by persuading a DWP decision maker the requirement is not "reasonable ... having regard to the person's circumstances" (Regulation 3(4)(a)).
In practice, I think you either have to attend or be sanctioned.