It was likely the Body Flicker.
Rin comments on it being "So Fast!
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But that doesn't make sense because FTG is instant, therefore, his FTG used on the Jonin fodder couldn't have been faster than the "FTG" used to save Kakashi unless he flickered there. Seems to make the most sense tbh, along with the smoke being an indication of the Flicker.
Of course, we also have that Jonin fodder realizing that he was indeed, "The Yellow Flash" after he warped behind him as opposed to flickering to save Kakashi, so him using the Body Flicker makes the most sense.