I do not think that's an entirely fair portrayal. It's not as if Klopp was unaware of or completely dismissive of Trents defensive limitations. How could he be? The argument was always his production in the attacking half more than outweighed his errors leading to goals, and given he has broken nearly all PL records for a fullback it's a strong argument.
This season the balance of the side all went to shit and Trents production from the wide areas nosedived, causing an eventual redesign. But it's fair to say the fullbacks in Klopp's original 433 became far bigger stars than the grunts in the middle of the park, given their effectiveness and attacking licence. Getting Trent into the middle was always going to require a greater shift than just sticking him in the Henderson role, which would have restricted rather than enhanced his talents.
I would say that Klopp could have started this this season with a new system, or found a solution far quicker than he did once it's clear it wasn't working, but I don't think we need to look back on previous years with any regret at Trent being at fullback. It's largely been hugely successful.