Russia can't afford to discredit itself abroad, nor can Putin's ruling United Russia party afford to discredit itself at home four months before the next polls, by threatening overwhelming retaliation against Ukraine if it attacks Moscow's Victory Day parade only to symbolically retaliate or do nothing at all.
The Russian Defense Ministry
warned local civilians and the staff of diplomatic missions in Kiev of their country's plans to launch a massive retaliatory strike on the city center if Ukraine goes through with Zelensky's
threat to attack Moscow's Victory Day parade on 9 May. This was followed by Russia
announcing ballistic missile tests from Kamchatka from 6-10 May. Shortly afterwards, the Russian Foreign Ministry
reiterated the Defense Ministry's warning, thus ensuring that the world is aware of it.
This threat likely isn't a bluff for three sequential reasons.
The first is that Russia wants to deter Ukraine from attacking Moscow's Victory Day parade for self-evident reasons, both relating to optics and the security of its VIPs, to which end it threatened overwhelming retaliation if this happens.
The second reason is that Russia cannot threaten such a response without actually going through with it if provoked, otherwise it would irredeemably discredit itself, and more audacious attacks would then likely follow.
Comment: It appears that Russia is truly drawing a red line that they mean to enforce. The only question remaining is, just how insane is Kiev feeling right about now?