The December 8th date is known as the "Safe Harbor" deadline. It was created after the disputed election of 1876 to prevent a situation where states submit multiple competing slates of electors. If a state certifies their results before that date, then that slate is final and there's no possibility of competing slates of electors:
Federal law (3 U.S. Code § 5) frees a state from further challenge if it settles legal disputes and certifies its results at least six days before the Electoral College meeting, which occurs this year on Dec. 14.
“Whatever final decision that state reached by (Dec. 8) — that’s conclusive and final and binding and nobody has any right to second guess it,” explained Adav Noti of Campaign Legal Center, a nonpartisan, nonprofit organization that focuses on voting rights, campaign finance, anti-gerrymandering work and government ethics.
If the deadline comes and goes and state officials submit conflicting electoral votes, Congress must agree on which to accept, according to the Congressional Research Service. If the U.S. Senate and House don’t agree, the votes certified by the state’s governor win.
Safe harbor deadline: Here’s why Dec. 8 matters in the 2020 election
If a state does not certify before then, there is the possibility of the legislature and the Governor submitting different slates of electors, requiring Congress to decide which one to accept. If the results are certified before then, however, there is no issue and the results will be final.
The Atlantic article poses a scenario where Congress would deadlock on which slate to accept. The wikipedia article on the Electoral Count Act goes into a bit more detail on this issue. The issue is whether the governor's tiebreaker applies just to a scenario where there are no electors appointed by the safe-harbor deadline, or also to a scenario where there are multiple slates appointed before the deadline.
So if there is only one certified slate of electors, or if no slate of electors is certified by 12/8, then the slate of electors certified by the Governor will win. The deadlock occurs if 2 slates are "certified" before the safe harbor deadline. In that case, there are multiple possible interpretations on whether the Governor's slate should be accepted, or whether neither should be.