Plasma is a superheated gas who's electron's pop off due to the intense heat, lightning is technically a form of plasma

So yeah, sanji actually generated plasma or technically turned the air around his leg into plasma by overheating it
logically and scientifically thats possible... but you don't realize how hot a substance must be to turn into plasma

air need over roughly starting from over 5k to tens of Thousands of degrees

it might be an aesthetic choice but dont make sense out of it or you’ll shit on akainu's magma.. someone who is directly superior to sanji in strength and ultimately in heat game as well