Can attest to this. The door going into the secure area is the most secure part of a SCIF and always has been. Some companies do the LAN inside the SCIF a bit better than others, but generally its not significantly any different from what is otherwise corpnet on the outside, and tends to be weaker because there (not so rare to find) isn't a corpnet on the inside.

As the article mentions, no new attack vectors were found, but just the scale of it is the concerning part.