I don’t understand how plastic bags for “100 years at current rates” is an argument FOR its longevity/being fine?

Yes… I don't think the author is thinking on a long enough time horizon.

I'm curious what happens to these landfill sites in 100, 1000 and 10k years. Do they inevitably break down and leak into the water table? Do they eventually stop off-gassing and become inert? Do they become fossilized layers of sediment on a long enough horizon? If we maintain our current waste levels, how much land is required by the year 3000?