MRSA has some benefits to consider alongside its detriments. It is capable of surviving on surfaces so you could apply it to a surface that your target interacts with and be relatively confident that it will be viable when the target contacts it. It's common, so it's not some sort of exotic bio-weapon that screams cloak-and-daggers: you can get MRSA in a hospital even. I think it's pretty resilient too so once your target has contacted the MRSA simple hygienic practices wouldn't neutralize it like taking a shower or bath.
But as far as I know it is very treatable by knowledgeable medical professionals if caught early so in that sense it's not a guaranteed attack-vector and a poor choice for an assassination. Depending on a target who doesn't visit the doctor until too late, maybe less of an issue.
It also colonizes a large fraction of hospital workers so unless you injected it into the victim or got them to inhale a large number of aerosolized bacteria, it won't be effective.
Assassination seems unlikely here to me. The presentation of sudden illness with multiple agents flu B, MRSA, and strep all at the same time is unusual. The presentation probably IS consistent with a sudden exposure to a very high number of infectious organisms into his lungs which could overwhelm the immune system and potentially lead to death. It wouldn't be impossible to orchestrate but it sounds difficult, finicky, unreliable, and tough to correctly execute without collateral damage. It would almost certainly require a state actor.
It's also plausible he contracted influenza B and suffered from an opportunistic infection of the lungs.
By Occam's razor I think the latter is quite a bit more likely.
