That is correct as LTIR does not accrue, so $10.55 mil [Horton + Clarkson] total is same on day #1 of the season and Dec 1st, but the cap hit for a player in year #1 goes up significantly if they sign late.
If Marner was signed in the offseason they could use the full $10.5 mil LTIR, however by Dec 1st a $10 mil AAV would have a $14-15 AVV cap hit.. Willy's $7.5 mil AAV contract became $10.27 mil in year #1 and $6.96 in all the other years due to how the CBA handles these late signings.
So Leafs could need to find +$5 mil more cap space above LTIR if Marner held out until Nov 3oth.
PS. The Ottawa Senators acquired forward Ryan Callahan and a fifth-round pick in 2020 from the Tampa Bay Lightning on Tuesday for goaltender Mike Condon and a 2020 sixth-round pick. Callahan announced earlier this off-season he will not play again after being diagnosed with a degenerative back disease. He will join Marian Gaborik and Clarke McArthur on long-term injured reserve with the Senators, raising their LTIR total to
$15.3 million.
Maybe Dubas should have helped out TB and acquired Callanhan's LTIR dead contract [$5.8 mil AAV LTIR] also to up their own LTIR by +5.8 mil to allow Marner to sign by Dec 1st.