Trump allowed Pence to be lied to because the WH knew about this weeks before Pence did. Pence literally found out by watching the news.
That Pence only learned about this in the media we knew already.
What I think Abramson is alleging is that Trump was aware of the content of the call between the time the call took place in late December and January 15, the day Pence went on the Sunday shows to publicly deny Flynn talked about sanctions with the Russian ambassador.
Based on the previous WaPo report, the WH was only informed of the actual content of Flynn's call on or shortly after January 23. Perhaps I'm just reading it wrong but "Trump was in fact fully briefed on the content of those conversations that Gen. Flynn had with the Russian ambassador" doesn't suggest to me
when he was briefed.