I'm not sure that follows. What's more, if people generally use that reasoning, and EFF is wrong (or lacked due diligence in such an estimation), then people will now be able to cite both SIAI's and EFF's actions when using such reasoning.

Perhaps you weren't aware that EFF is a "digital rights advocacy and legal organization"? If we can imagine that Bitcoin might have legal issues, how could that have escaped the people at EFF? On the other hand, SIAI is hardly known for its legal expertise, so the fact that it accepts Bitcoin offers little additional evidence for Bitcoin's legality.