Don’t think late presentation STEMIs are that common to begin with for your argument to have logical sense, this is the worst form of a “heart attack”.
From this single center study presentations > 12 hours only comprised 10%.
https://www.ingentaconnect.com/content/wk/jcarm/2017/0000001...
Other weekday numbers will also have delayed presentations included so you can't just "subtract 3.1" from one day and declare statistical insignificance.
What you can do is subtract it from every day as we know that 97% of STEMIs present within 12 hours.
As this is just an abstract we don't know what the authors did in this particular example but it's not the first study to suggest Mondays have the highest ACS rates.