Network difficulty adjustment is there to ensure we get a new block mined, on average, every 10 minutes. It does not impact the cost of a 51% attack, just the block mining rate.

As emissions drop, less money is spent on mining and a 51% attack becomes cheaper.

When China turned off mining, mining temporarily became more profitable as it took some time for miner spend to get back to a equilibrium state (where miners, in aggregate, spend nearly the entire block reward on mining costs).

It did temporarily get 'cheaper' to conduct a 51% attack (although it was still so expensive as to not be viable - due to the currently high block reward). This wasn't because of the difficulty adjustment though - that just maintained the average time to mine a block at 10 minutes.