You gotta watch the film more in detail - Boruto only cheated in order to gain his father's acknowledgement. Rather than him actually needing to.
Though he kinda lost himself mid way and became engulfed in the power of the cheating.
I haven't watched the movie either. I read that portion of the manga for the first time a couple days ago. During the second phase of the exam, Boruto was being overwhelmed by his opponents then used the devices to dish out water & lightning. Just as he did later with Shadow Clones against Shikadai.
If these were things he could easily use without the device then why would he use it? To conserve chakra would be a good reason, but that was never stated nor implied so it's headcanon at best.
We know at max he could make 3 clones, so him using the device to create more clearly shows that he relies on it to perform beyond his limits when he's in a pinch. Based on this, he couldn't perform the Water & Lightning technique as easily through his own efforts at the time.
Furthermore, he also used the device to make it look like he mastered Rasengan to impress Sasuke, even though he could only make a baby Rasengan on his own. Which adds to the above.
Later when he's sparring with Naruto he uses both elements with ease. Based on his improvements with Shuriken Jutsu against Shojoji aswell as Rasengan against him & Naruto, it's clear that after the exams he trained offscreen to gain better usage of his arsenal, hence his Shadow Clone limit also improving to 4. Naruto even specifically mentions his improvement with Rasengan came from training.
In conclusion, Boruto used the device to produce what's beyond his current capabilities, yet the anime has him doing said capabilities without the device, easily, and at a younger age.
If academy Boruto was really Chunin level as Kakashi said, then he wouldn't have needed to cheat so much during the actual Chunin exam which he undertook months after he became a Genin.
Inconsistency.
