For the same reason, the existence of ancient life on Mars would not show that life is likely to arise on Earth-like worlds, since one could not conclude it was an independent event.
By what mechanism is an impact giving some debris on Mars enough delta-V to reach Earth, without obliterating Mars and also sterilizing the debris due to heat and plasma?
Impact spallation.
When the strong shock wave generated in rock by an impact hits a rock/atmosphere (or rock/vacuum) surface, it is reflected, producing strong tensile stress. This causes very large acceleration while not subjecting the spalled layer to extreme temperature or pressure.
This is the same process that HESH (High Explosive Squash Head) antitank rounds use to defeat tank armor without penetration. The explosive squashes against the armor before detonating, launching a shock through it causing material to spall off the inner surface at high speed. The shock here is weaker than in the impact case, however.
any would-be hitchhikers would need to be able to survive extinction-ish level impacts on Mars, survive through the vacuum (e.g. months), and then survive atmospheric re-entry and landing.
and then be able to survive in the new ecosystem
https://www.youtube.com/watch?v=by1xpy8ob8E (starting around 5:50).
The evidence for life on mars is based on carbon nodules in rock, and things like CO² and methane gas,and some other chemistry and mineralology. The main one are the carbon nodules, which are ubiquitous in many sedimentary rocks on earth, though in the samples that I have of earth rocks, there are very often macro fossils that are naked eye identifiable There are countless miles of cliff here where everything has a fossil, but complete specimins, and soft tissue fossils remain rare, here, so the chances of such a thing making it to earth from mars, schould it exist in the first place are almost zero, though the martian ice caps could concievably hold meteors with earth life fossils that are naked eye visible and identifiable.