Doesn't this already require to be "on the other side of the airtight hatchway", or am I missing something?

The Marshal.load docs explicitly have a warning that you should not pass it untrusted data: https://docs.ruby-lang.org/en/master/Marshal.html#module-mar...

Yes, but that doesn't mean defense-in-depth isn't worth doing. The article discusses how known gadgets were removed in the past.