Ejected into space from impacts, not entering the atmosphere from space.
" Furtherwork is needed to investigate the existence and abun-dance of microbial life in the upper atmosphere, partic-ularly at the altitude considered here,∼85 km, at which Earth-grazing object would avoid significant heating. "
So, their argument hinges on the assertion that object grazing that high up would not get that hot. They argue that a porous object could graze very high up in the atmosphere, pickup any microbes up there in their pores, and in very deep pores >10cm where the temperatures may not get so high the microbes may survive.
The paper isn't peer reviewed and I'm personally not equipped to say if their reasoning is fully sound. It's a pretty short paper and pretty light on details. They are just pointing out that it's a a possiblity that needs further investigation.