Never heard this before, in any case not for this route. Can you link the blog where you saw this?
You can book all the required passholder reservations (Thalys Rotterdam-Paris + TGV Paris-Barcelona) in one go through NS International on the phone (they might charge you a considerable fee of about €7.50 per ticket - check as I've been given contradictory information) or through the Interrail booking portal (€2 fee per ticket), and they will be confirmed directly.
The only thing I can imagine this could refer to was that up until December reservations for the Paris-Barcelona TGV booked through the interrail portal had to be delivered by post. Those are now directly generated as e-tickets too.