Poster-boy example for a "How Not To Spin the Casimir Effect" education campaign.
The plates are in a "vacuum" as we normally consider it. In the skinny volume between the plates there is less pressure than in the "vacuum" surrounding the plates. Attractive forces between the plates can be eliminated. Thus, the "vacuum" wasn't empty.
Your attempted rebuttal simply misses the point. It's like those "You still have a designer" rebuttals to any scientific experiment conducted in a lab by a human.