There's no legal possibility of an extension to the transition period as such, without an amendment to the withdrawal agreement or something in a future relationship treaty.
When Cummings went on his Barnard Castle trip, Borisbots claimed that he had to be saved because only he could stop the UK agreeing to an extension to transition. This sounds like a rehash of that argument, although it's now legally weak as the extension deadline has passed.
And as Dom (no, not that one) points out, the reference to "Remainers" is gratuitous gibberish
nb this means that Ireland will participate in criminal law aspects, NOT immigration aspects, of Schengen 1/
2/ This is very similar to the UK's partial participation in Schengen - which will finish at the end of the transition period, ie at the same moment when Irish partial participation *starts*.
The author of this tweet is being misleading, in order to defend his or her previous lie. The current tariff on soya sauce from Japan is zero, due to the EU/Japan FTA which the UK applies until the end of the year. The UK and Japan do not currently trade on WTO terms.