Notice Kuvira has distanced herself from her element as much as she can, in every way that she can short of bending something else entirely. She fights while being entirely uprooted from the earth on her train, one of the more critical fundamentals Toph promoted. Even when rocks are thrown at her, she either ducks, redirects or slices them with metel, but never directly confronts or makes contact with the earth. Also, her movements are much more agile and swift than regular earth bending, which is particularly disadvantaged at delivering speedy attacks both on target and with enough force to be effective.
And what’s more, she’s not using regular earth, she’s using metal exclusively , which is arguably at least one or two steps away from original earth.
And something of interest that I hope is developed by later episodes, is that to force her earth bending opponents into submission, she blinds them and it works, which shows that she either does not humor the idea that blind earth benders might be a threat (or as we may find out, a very specific blind earth bender) or that she doesn’t consider any of her natural rock-bending-earth-benders adequate enough to take full advantage of their element. Which is probably more likely since the last moments of the fight were of her literally looking down on her defeated enemies with contemptment and ominous music.