Good point, except for this one problem:
Just as we do not know that our microorganisms here on Earth could infect extraterrestrial species, we also cannot know that they could not. It is also reasonable to assume that the process of evolution is present in all life (if to varying extents). Microorganisms are AFAIK the most quickly-evolving of Earth’s life forms, so how long would it take for them to evolve to live on and inside of alien life forms? We don’t know that they can, but we don’t know that they can’t…and we do know that they have evolved to live and thrive even in extreme environments here on Earth.
I’d say that we shouldn’t underestimate the robust nature of life on the microscopic level.