Actually I’m fairly confident they could 51% Bitcoin if they wanted to right now, but there isn’t currently an incentive to do so. They personally control 30% (known) of the hashrate in their own farms. It has been estimated they control close to that amount by proxy via their pool. So technically they could do it, but that would be bad business sense as Bitcoin is a major cash cow for Bitmain.
Instead they have done “other” things like the empty block debacle last year that created quite the profitable mess for some.