Agree to disagree. Seems rather superficial analysis to me.
For forwards to be in the box, they need to have faith that the wide players will deliver decent low or high balls. I don't remember when the last time was when we had a team capable of doing that, but it hasn't been through Martial's 5 years at United that's for sure. Neither AWB nor Shaw/Williams are doing that consistently or with decent quality like Liverpool's fullbacks are doing for example. And on the rare occasion when a good ball was put in the box, Martial was there to meet it. Martial, like any forward, wants to score goals and he certainly would be more inclined to spend time in the box if decent deliveries were forthcoming. But it's kind of bizarre to insist he should be doing it regardless even when they aren't.
His analysis is also disregarding of Martial's perhaps strongest qualities, which are speed and running with the ball, and his natural inclinations. This type of forward thrives on counters, being on the shoulder of defenders and being fed by through (or over-the-top) balls, or dropping deep and picking up the ball to run at defenders. Even if we were providing decent service into the box, Martial will not become Drogba or Inzaghi. I'm alluding to the type of forward, rather than quality of forward here.
So for Martial to thrive on his strengths he needs the appropriate service. Now there's a stat on Pogba's performances page today which says he has the most through balls from any United player this season (3), despite not playing since September. The rest of the midfield without Pogba simply cannot provide the type of balls that forwards like Martial and Rashford can thrive on.
Finally, I think both Martial and Rashford are having decent-to-good seasons and criticism of them is over the top. Martial has 9g 3a in 20 league games with only 1 pen, while Rashford has 14g 4a in 22 league games with 5 pens. Considering they both went through long periods of playing without each other's support up front and pretty much all season without any decent service from the midfield, I reckon they're doing well. Not that they don't have a lot of room for improvement of course, but expectations of output must be grounded on the reality of this team's limitations.