The 2017 General Election with Theresa May as Prime Minister saw the Tory party getting the then largest share of the vote since 1983, ie 13,636,684 votes, which was 42.4% compared to Labour's 40.0%. It just didn't translate into seats.
en.wikipedia.org/wiki/2017_United_Kingdom_general_election
The 2019 General Election with Boris Johnson as Prime Minister saw the Tory party slightly increasing that to 13,966,454 votes or 43.6% of the vote.
But the Labour Party crashed under the anti-Corbynism and the vote distribution translated into Tory seats (as it should, because that's what the Tory campaign was targetting).
en.wikipedia.org/wiki/2019_United_Kingdom_general_election
So "I have a mandate and Theresa didn't" is mince.
It's all just words to Johnson.
A "reason" is needed for something, and he just pumps hot air out of his gob until his listeners have lost the will to pursue.
To be fair to him, it's a technique that's worked well through all his relationships. Crashes badly when he meets a questioner who doesn't give up, though, as he learned yesterday at the Liaison Committee.