Correlation doesn't imply causation. Other circumstances changed. For example, Corbyn has found himself with a de facto majority, what with one Tory defecting to the Lib Dems and 21 further Tories being deselected. That gives him the power to agree to the general election at a time of his choosing, rather than Johnson's.
So, we have two hypotheses here:
1. Corbyn ran chicken because of something Tony Blair, who is if anything regarded as a worse enemy to Labour than the Tories are amongst Corbyn's wing of the Party, said on Friday about how Corbyn was going to lose.
2. Corbyn decided that he may as well milk his moment of de facto power for a while, so as to frustrate Johnson in particular on the question of the October 31st date, before agreeing to an election on his terms.
My money's on the 2nd one.