Considering Facebook owns WhatsApp this isn't surprising, they're definitely happy to share this information (ever seen someone in your FB "suggested friends" list after messaging them on WA?)
If they had pushed a different version of the app to Russia or there was a way of breaking message encryption I'm sure someone would have noticed by now.
[1] https://www.heise.de/mac-and-i/meldung/Entwickler-Facebook-k...
Corporate statements about the security and privacy of their products usually leave open the possibility that they engage in lawful intercept. Even if they don't they can always just lie and use the fact that they have national security letters barring disclosure as legal cover if it ever comes out.
https://www.theguardian.com/technology/2017/jan/13/whatsapp-...
If you're a user of WhatsApp, make sure you enable the two-factor authentication PIN, too, because otherwise it should be relatively trivial for your own government to gain access to your account by using your number with the local carrier's help (it's also possible to do it without the carrier's help, given how broken the security of carrier networks is). Many Telegram users were hacked this way by the government in Russia, too.