... wasn't one of the strong points of plate armor being able to deflect arrow fire? I would understand if they started killing horses left and right and the knights mounting them broke something in the landing, but beyond that...
However, the tactic itself isn't bad. Use dud bonbards to fool the french into thinking that the weapon is useless beyond riling up horses for when the bonbards are actually loaded the day after. Meanwhile the foolish knights charge ahead of time in a manageable number that can be dealt with longbows and ol' knight lances. Nice.