There is lots of discussion on this in previous posts and in some FB group stuff.
The difference is the drag tube. Highlanders have a drag tube running under the fuel tank to deal with the flex.
On FB in 2020 Brandon mentions the additional weight, complication, and capacity loss of installing drag tubes to allow the change to justify the decision of this change not being worth it.
It can be done and there are a few KF with aluminum tanks however this is not an easy or approved change.
Also to note - The highlander tanks are not a drop in fit for the KF SS wing profile.