The parent isn't claiming that it's legal to take something so long as you have intent to return it. Just that the standard definition of theft requires "permanently" as an element, and maybe taking something temporarily would be some other crime.

See the model penal code[1] "(1) "deprive" means: (a) to withhold property of another permanently or for so extended a period as to appropriate a major portion of its economic value, or with intent to restore only upon payment of reward or other compensation"

See [2] which mentions the "permanent" requirement three times.

[1] https://archive.org/details/ModelPenalCode_ALI/page/n207/mod...

[2] https://www.findlaw.com/criminal/criminal-charges/theft-over...