I'm hardly an environmentalist, but for a personal example, my home happens to have a well for drinking water, a perturbation of the environment that's perfectly sustainable, we use less water from this well than the watershed on my property restores to the aquifer. This is an acceptable perturbation IMO. That well is contaminated with polyfluoroalkyl substances from Scotchgard-treated leather scraps. It was too expensive to dispose of them safely, so Wolverine leather buried truckloads of the toxins in a local swamp. This 'perturbed' the environment, turning the place into a Superfund site, making the entire aquifer toxic for decades to come, and should not have been done.

If underwater mining just makes a depression in the seafloor with little other effects, that's probably an OK perturbation. If it releases heavy metals into the water that travel with the currents for miles and miles, killing much of the wildlife, that's an unacceptable perturbation.

Deep sea mining doesn't really do much for either.

Nodules are never going to provide enough to meet demand, so mining nodules doesn't get rid of open cast mining. We don't know what the impact is of mining deep sea nodules, but it's likely to be pretty bad.

If open cast mining had a history of attempting to fix the mines (eg re-wilding) when they'd finished we could trust them a bit more when they want to mine new untouched wilderness. But they mostly don't have that history.

EDIT: for an example of the harm already caused: https://www.nature.com/articles/d41586-019-02242-y

My understanding was that this one area is estimated to have more rare metals in these nodules than all land based deposits combined.

I think the reason you can't wrap your head around them is because, with possibly a few individual exceptions, 'any human effect on the environment at all' isn't actually what they're opposed to.