It’s an interesting idea. Filtering and searching are similar concepts but with a key difference. Filtering: taking a large set and producing a subset. Searching: starting from nothing and producing a set of matches.
Both take a query and produce a result set, but filtering is starting with the images already visible, while search is starting from nothing visible. The app filters rather than searches because then it’s just hiding images from view and everything else works the same way as if there was no filter.
Nitro stores metadata for files it has seen in a database. However, because the file system can change behind its back, it cannot be sure that the files that it was are still there (or if new ones have been added).
So for a search, it cannot simply consider its database. It would still have to walk the entire tree. It would not need to display all the thumbnails, that’s true.
Then, when there is a match, it would need to find a different way or UI to show the images, since they can be scattered throughout the tree and that would break the assumptions of where the images are coming from. So, that would require a different type of view (in the way that the Finder has a different view when you search there).