Sorry, but I don't agree that this is serious, they were both in this together, but for whatever reason (Carrie maybe?) Boris sacked Cummings, at a time when Dominic thought he was 'fire-proof'...end of any 'seriousness' there maybe!
This is now a round of.... 'get you at playtime' from Cummings, he's telling Boris, "I will show everybody your weaknesses"... trouble is we all know Boris's weaknesses and most people love him for it. The other problem is that Cummings shows himself to be vulnerable, and not half as clever as he wants people to think he is... i.e. he clearly was not able to convince Boris and/or save his job.
As for telling lies... well goodness me, shock, horror... politicians and their advisors sometimes tell lies...would you believe it?