Is there a market out there where I can bet all my money against this claim? The idea that this will lead to a spam wave destroying Reddit is wishful thinking on the part of aggrieved moderators who see themselves as forming a Thin Blue Line against a tide of chaos.
Having given up moderation ages ago, I have no dog in this fight. I'm sitting on the sidelines watching the drama with a bowl of popcorn, just like everyone else.
If there's a detailed argument saying that basic moderation (eg. obvious spam removal and not "I want to ban everyone who's ever said anything nice about Trump") will take say an order of magnitude more human effort without 3rd party tools I'd be curious to see it. If there's a persuasive claim along these lines I'd shift my beliefs, but I haven't seen it so far.
https://github.com/toolbox-team/reddit-moderator-toolbox
I don't know if the proposed API changes will affect this specific tool, but I doubt moderators would be nearly as up in arms if their tools were going to keep working the way they had in the past.