The disagreement, from what I can tell in this brief summary with no opinion out yet, doesn't seem to be over the "interstate" part so much as the "product" part. The federal government's Constitutional commerce-clause power almost certainly includes the ability to regulate platforms like this Goldman platform, since it's heavily tied up in interstate (and international) commerce. But the wording of this particular criminal law, the Economic Espionage Act, arguably applies only to "products" sold in interstate commerce, not internal software that isn't for sale anywhere, even if used internally for purposes closely tied up with interstate commerce. At least, that's what his lawyers appear to have successfully argued.

I haven't read the court's decision to see exactly what part of the statute the guy was charged under and the court said did not apply, but the wording of the part that looks like it would be the right part says:

   Whoever, with intent to convert a trade secret, that is related
   to or included in a product that is produced for or placed in
   interstate or foreign commerce

I'd expect the internal software to qualify under the "related to" part, as it is related to their trading product which is heavily involved in interstate commerce.

I read it as saying that the trade secret has to be "related to" a "product that is produced for or placed in interstate or foreign commerce". So if their trading platform isn't "a product", and isn't itself "produced for or placed in" interstate or foreign commerce (but only used internally), it may not apply. Depending on what exactly "placed in" means; it seems the court interpreted it to mean that the product had to actually be put into commercial channels for sale, not merely used internally for commercial purposes.

'Commerce' here means something sold in the open market, not something used privately as an aid to market trading. Goldman's trading software was not for sale to others, so it had not been placed into the stream of commerce.