Mining a block that conflicts with an existing block but then becomes part of the chain with the "greatest work" is called a blockchain reorg. It is not a frequent occurrence, but bitcoin's design can handle that. And that attacker can only double spend their own transactions.

Censoring some (or all) transactions is definitely something a miner with 301 EH/s (of the total 601 EH/s .... which is the existing network's 300 EH/s plus the attacker's 301 EH/s) is certainly capable of doing. It's the finding and operating the 301 EH/s hashrate that I doubt even the most motivated nation state would be able (nonetheless willing) to pull off.