If 20,000 humans weren't significantly harmed from disease because of this release, but 70,000 frogs faced starvation, which would you choose?
Can you assure that saving this 20,000 humans will not mean one million humans starving and dying later when their main source of proteins (fisheries) will vanish forever by lack of mosquitoes and introduction of mosquito fishes (those pesky fish egg eaters)?. Wouldn't choose to save one million babies, you? Wow, that's evil. Think in the babies, people!
Even us filtering water for drinking has downstream effects which may ultimately cause harm which can't be entirely unproven. I guess we should just stop drinking clean water!
There are tons of things we just ultimately choose to do or allow to happen which can have potential issues we may never even know are happening until its way too late. Ruminating on maybes we have no way to even measure or know or simulate isn't exactly useful. What are the likelihoods of those outcomes? Pretty dang small based on our current knowledge? What are the likliehoods of tons of people dying or being seriously injured from these diseases? Extremely high?
If we lose those frogs, it most likely would cause a cascade across other species. There are lots of things that eat frogs.
In the end, whose lives do you prioritize given the lack of information today, and the lives at risk today? Deciding to do nothing still costs lives, and potentially drives us further into the bad state area.
They're only applied to thing we don't want to do.
In reality, it's always true that we don't fully know, so applying the argument always means "let's not do this".