PG agrees that you can't conclude that beautiful writing, writing that seems plausible and convincing, actually is. Sophistry is writing (or speaking) that seems plausible and convincing, but is false. PG is not saying that's not possible.
What he's saying is that writing that is ugly is highly likely to be wrong. Which has nothing to do with sophistry.
It might be that it’s hard to create sophistry accidentally in one’s writing, but it’s certainly a possible - and common - trick.
The danger is when you convince yourself that what you’re writing isn’t sophistry… because - after all - it looks good.