It is reported that the passage was secured by a US P8 anti-ship aircraft.
That seems pretty significant. It seems unlikely the ships would have been targeted by Russia if they were flying the flags of their country of origin, but to have an armed escort is definitely a huge deterrence. I have to wonder what sort of response it'll garner considering all the other 'unprofessional acts' Russian aircraft have been employing against unarmed US aircraft lately.