Damping off of tomatoes is caused by several genera and species of FUNGi.
https://en.wikipedia.org/wiki/List_of_tomato_diseases
Scroll down to fungal diseases and look for damping off.
The only way to prevent it is by sowing seeds in artificial mix where there's no soil,aka dirt. You can also sterilize other mixes in the oven but hardly anyone does that these days.All b/c those fungal pathogens are soil borne pathogens.
I posted last week here that while the brown constricted area at the soil line occurs with newly germinated plants is well known that it can occur up to seedlings 4 to 6 inches tall as well.There's a nice picture of that in one of my tomato disease monographs.
Yes cinnamon has some antibacterial properties,I never used it, and here's why.
Back in the early 80's when I started at the AOL tomato forum there was a woman who posted there who was a technician at some company, I can't remember which one,and the use of cinnamon for damping off came up.
She asked her boss if she could do some experiments with that,used CONTROLS, she did doit and it clearly showed that cinnamon was not effective in preventing damping off.
Consider England who controlled the spice trade in the mideast and south sea islands. When they brought all those spices back, yes,they were used in baking and otherwise,but also to help preserve especially meats of all kinds by coating them with cinnamon and allspice,etc. to help preserve them.
One of the History magazines I sub to is published by the BBC in England and they always publish some recipes from way back.
Just what I've always wanted to taste,eels preserved in spices.
Summery,use the correct seed starting mix and you won't have damping off.
Carolyn