Attack is still not infantries strong suit, they have better defensive stats
I suspect most people took mech inf over mot inf so they needed a buff to make it more even?
This has an aspect of realisum attached with US marines in WW2 who actually used mortars.
As seen in "The pacific" (miniseries)
Yet if you attack with a 10 stack of marines, against a 10 stack of infantry, the infantry on defense will attack first dealing massive damage, then deal even more damage when attacked with its defensive stats. Its only in the second attack the marine's defensive stat will be of use and at that stage its already lost both two battles.
Marines are shit, mech infantry are more often used because they have defensive stats, more hitpoints against aircraft but the tradeoff is they are expensive and hard to replace. Where marines are sitting ducks, not saying you can't use them but the cost of lvl 2 army, level 2 docks, and recruitment office far outweight a lvl 1 army base yet the unit maxxed out is pathetic in an offensive role.
Seems Marines are a gimic, a single unit to capture a tile to build a pontoon bridge. An engineer unit, not a combat unit.