The only problem is the national security angle
Yes, if these companies were lying about not having found or been impacted by the alledged backdoored hardware, then you could expect an investor lawsuit.
However, you could also see the Feds moving in during such a suit and filing motions to suppress the relevant evidence on national security grounds. I've never heard of this being done during an investor class action, but it certainly is done during other civil actions (Recent suits to gain access to FISA court rulings and to divulge national security letters being served to various parties, for example), and in those cases these motions are most often accepted.
Its sad that in this day and age we have o worry so much about who is lying to us. However, these major tech companies created these problems for themselves, when they failed to stand up to federal orders in the past.