You might notice I primarily make only positive points on my blog. In that passage, I'm describing the WSX-25 as it is sold, and explaining the hardware decisions that Apeks made specific to their rig.
Here's on the forum, in this thread, I'm advising and answering questions on sidemount config, principles and strategies.
Very different writing goals, which emphasis different issues. Either way, the two statements aren't opposing.
Serious divers seeking incorporate sliding D-rings would do so for one reason.
Novice, underskilled, divers would seek to add them for a subtly different reason.
Both reasons are an attempt to 'improve performance'.
The serious (skilled) diver might seek improved equipment performance, building on top off a solid foundation of skillset.
They would manage perfectly fine with fixed D-rings, but might identify a benefit... specific to their unique diving needs... in swapping to a sliding D-ring.
In contrast, the underskilled diver would be attempting to substitute an equipment upgrade for a skill deficit. They'd be struggling with basic sidemount foundations; such as cylinder manipulation and boltsnap use. They'd look to circumvent that skill deficit by finding an 'easier way' through equipment changes. This approach often only serves to further amplify their underlying skill problems and cause more problems and frustration.