It's not that improbable. They've found meteorites on Earth that have been identified as having come from Mars based on their mineral contents.

Sure, I was referring to the part where the microbe survives the initial blast, the passage through space, reentry, and landing, and happens to land in a spot where it can survive.

According to some research, microbes survival in meteorites is very likely (if they exist)

https://www.ncbi.nlm.nih.gov/m/pubmed/11543506/

I know enough biology to be skeptical of papers like that (although, I admit I was skeptical when papers from the 50s predicted interplanetary mass transport, and didn't change my mind until we found Martian meteorites). I'd like to see actual evidence of this.

I actually think the rock getting into space with the microbe embedded in it is not that unlikely (many rocks in the earth are laced with bacteria), nor the space transport. I think the excessive heat of rentry, the force of impact, and the difference in target ecosystems are what are going to kill the microbes.