You're confusing CPTSD and PTSD. Your link talks about EMDR for PTSD, not CPTSD. The post you're replying to is talking about CPTSD. They can be quite different.

As the sibling pointed out, C-PTSD and PTSD are not the same thing. When EMDR is incorporated for C-PTSD, there are some aspects that are modified based on the trauma history, and some cases where EMDR cannot be attempted until dissociation issues have been addressed.

It is this need for modification and the prerequisite work (sometimes the long/hard kind of work) that makes it less clear-cut as a treatment for this form of trauma.

The EMDR practitioner I worked with made me aware of this as did my regular therapist, and most C-PTSD-aware EMDR info you can find online will also explain this distinction and the necessary modifications/prerequisites.