It seems very unlikely - the researchers estimate a lifetime for phosphine of just thousands of years in the more temperate parts of the atmosphere, and just thousands of seconds nearer to the surface.
There is scientific evidence establishing that diatoms and other microorganisms are wafted up into Earth’s upper stratosphere and beyond 25–100 km above Earth’s surface.[1] From there, passing dust particles will transport a few of these microorganisms to the upper atmosphere of Venus.
Another transport route would be large meteoroids bombarding Earth with sufficient force to export Earth rocks and soil into interplanetary space.
[1]Alexa R. Van Eaton, Margaret A. Harper, and Colin J. N. Wilson, “High-Flying Diatoms: Widespread Dispersal of Microorganisms in an Explosive Volcanic Eruption,” Geology 41, no. 11 (November 2013): 1187–90, doi:10.1130/G24829.1; Milton Wainwright et al., “Isolation of a Diatom Frustule Fragment from the Lower Stratosphere (22–27 Km)-Evidence for a Cosmic Origin,” Journal of Cosmology 2013, volume 22 (August 9, 2013): 10183–10188.
