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.