First, none of the pools in the link you provided have 30% computational power (hashrate) not to say 50% and the pool with the highest hashrate is foundry which is currently at 26.9%.
I think you do understand the meaning of 51% attack
A 51% attack is a possible security vulnerability in cryptocurrencies that always happens when a single entity achieves control of more than 50% of the total hashrate of a crypto network.
I think he talks about how if these two pools are combined it is more than 51% that can be used to attack the network.
I don't know the reason why these two have more hashing power compared to other pools but I think the only reason why most of the miners mine on these pools is because of the pool fee they offer 0% pool fee.