I don't know how many casualties were caused on each side so it's hard to tell whether RA would have made a difference in this case, probably not, it was pretty bad luck to roll double 2 for resolution!
I think the lack of AT capability is not a huge loss, most marine armies of every flavour have limited AT firepower (scions excepted but they're not a "proper" chapter anyway
) and against guard with masses of 4+RA you're basically hoping for a few lucky kills here and there from shooting rather than any serious dent in your opponent's forces
in every game I've played with Steel legion, whenever I've taken a tank company, it's always been taken out by engagements, never from shooting.... hackdowns are the absolute best way to kill RA tanks, then they're very vulnerable to being picked off when broken
