Possible to flash custom rom via Flashfire on locked bootloader? See video inside

I know the answer is 99% NO but still how did this person do it? Does the galaxy use a different "kind" of bootloader that allows this?
https://www.youtube.com/watch?v=bZDMHktN3sc

has anyone with a rooted (but locked bootloader) G4 actually tried the latest flashfire with a G4 custom rom like resurrection?

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