It's an attack, so we should still take it seriously, but not an actual 51% attack. If that were the case they would already basically control the network and be mining empty blocks consistently (we're pretty far from that)

I don't know of a good place to read about it but here are metrics where you can observe the attacks when they happen. Notice the vast amount of blocks in between each attack (blocks in yellow). Many blocks are condensed into one so the graph makes it seem more severe than it is if you don't notice that.