Debris are a half-level hindrance.
I don't think that it affects fire from units at one level difference one from the other, in the case (here) when there is not a continuous slope.
I think that H10 Debris would hinder fire from/to H11 to H9, as there is a continuous slope, though - idem G11 to I10.
What would the errors be?
Half-level hindrances and (non-)continuous slope rules seem to solve the DRM question - but I know that I miss some aspects of the rule quite often.
Crest line is in the wrong spot--I don't think higher crest lines are supposed to be in hexes with a lower center, e.g. that crest line in H10 should be in H9 and I10. It turns it into a 'use your judgement because technically if the crest line is in H10, it plateaus I10 WRT G10. but I'm not always great with crest lines either, so Mister T may need to correct me