Unfortunately a football season doesn't consist of only one competition. You don't judge how a players season has gone and ignore everything other than the Prem? THAT is bullshit in my opinion.
Heres the stats for each player.
Kane -
https://www.transfermarkt.co.uk/harry-kane/leistungsdaten/spieler/132098
Martial -
https://www.transfermarkt.co.uk/anthony-martial/leistungsdaten/spieler/182877
Martial has played about 3 games more and scored 1 more goal. Not a massive difference but not half a match more.
The difference is Martial is having the season of his life, Kane is having a bad season and there's 1 goal in it. 17/18 season Kane got 41 goals. 16/17 season Kane got 35 goals etc.
Like I've said previously, no one is saying Martial can't maintain his form, especially if more top players join the club and he took more penalties but until he does it for 2/3 seasons, let's not pretend 6 goals since the restart have propelled him to being a better striker than Kane.