In reality its probably the complete opposite. The powers that be higher up in the parent companies have put pretty interpretable blockers on Mercedes or Ferrari supplying Red Bull.
Their options now appear to be only between Renault and Honda, both of which look unlikely, but mildly less unlikely than Mercedes or Ferrari.
If the Renault talks come to nothing I think we may see RBR and STR going to the FIA and asking for special permission to use 1 year old Ferraris, like Manor have this year. And I expect the FIA would allow it rather than lose 2 teams.