Seriously you could not make this up!!
Is it supposed to some sort of the (sic) threat?
It isn't a threat (nor is it made up).
Nor is it difficult to understand.
The rules/regulations regarding travel during the pandemic apply to EU member countries. The UK remains an EU member country until December 31st when they will be leaving the EU
Therefore the current covid travel rules include UK citizens - just as they include the French, Germans, Spanish etc.
Travel to the EU from outside the EU is currently not allowed (although there are some exceptions).
After December 31st the UK is no longer a member of the EU so the current rules will no longer apply.
It isn't a threat, it isn't spite. It is simply that by leaving the EU the UK will be subject to the same rules that apply to all of the other countries which are not members of the EU.
But that's what the UK voted for.
It's like cancelling your gym membership but still expecting to be allowed in to use the gym.