Rules are often repeated when relevant, particularly obscure ones like this. In all likelihood, it was solely meant as a reminder of this rule when it existed, and possibly to prevent anyone from arguing that “using” it “as a +4” didn’t mean it was a +4 and so it still couldn’t damage those weapons.¹
As it was probably just reminder text/confirmation that shatterspike really works like a +4 when sundering, yes even that way, we should probably not read anything more into it than that, and update shatterspike along with the rest of the rules about sundering magic weapons.
- One might wonder how you use a weapon “as a” +anything in that case, though, since the use of such weapons is identical to those with lesser magic, and indeed to non-magical weapons. But I never said it would be a good argument, only that people might make it. Cutting such things off at the pass is often necessary in rules-writing.