There are at least two possible Catholic responses to the charge that porneia has been mistranslated as fornication.
One possible response is the category argument. It goes like this:
When you condemn a category, you condemn the things in the category. If I told you it was a sin to have furniture, it would logically follow that I don’t think you should own a chair, because a chair is in the category of furniture. Now, the Bible says porneia is a sin. And there is evidence that fornication belongs in the category of porneia. (For one thing, Strong’s Greek Dictionary includes fornication as one of the meanings of porneia. For another, I think that dictionary bases its definitions on the way the words are used in similar documents from the time period.) It follows that fornication is a sin, because the category which includes it is a sin.
The category argument is true except where there is some reason to make an exception for some member of a category. I would have to see the whole essay to see if he has a reason for making fornication an exception, but I doubt he does, more likely he just asserts it.
Another possible response is the non-sequitur argument, where you demonstrate that the essay’s conclusion does not follow from its premise. (In Latin, “it does not follow” is “non sequitur.”) Here’s how that argument goes:
I admit the premise but deny the conclusion. The essay’s premise is that fornication is a mistranslation of porneia. Its conclusion is that fornication is okay. That conclusion does not follow from that premise even if the premise is granted. This is because fornication can be wrong for other reasons than just based on the word porneia. Fornication goes against natural law, for example, and there are other passages of Scripture that condemn fornication without using the word porneia.
You can believe that fornication is a correct translation of porneia, like in the category argument. Or you can believe fornication is a mistranslation of porneia. The Church does not require us to believe one translation is correct.
Catholic apologist Hubert Richards has an article where he uses the premise that fornication is a mistranslation to defend Catholic doctrine on divorce and remarriage. Here’s one particular line from his article: “The Greek word porneia that is used in Matthew 5 and 9 is in fact both more general and more specific in meaning than the English word ‘fornication.’ In itself it means simply ‘impurity’…and the context must decide what precise impurity is being referred to.” source
You can believe that fornication is a correct translation of porneia, like in the category argument. Or you can believe fornication is a mistranslation of porneia. Either way, the essay’s argument fails.
I hope that helps. Please let me know.