Deletion isn't really a good early option (you have none left), 'first do no harm' don't delete, send virus to the chest and investigate.
That bit out of the way, the author sounds quite knowledgeable as it is entirely possible that a string within the file matched a signature. As to how to check if it was a false positive, this is only possible since you backed up the file otherwise that isn't possible.
You could also check the offending/suspect file at:
VirusTotal - Multi engine on-line virus scanner and
report the findings here the URL in the Address bar of the VT results page. You can't do this with the file securely in the chest, you need to extract it to a temporary (not original) location first, see below.
Create a folder called Suspect in the C:\ drive, e.g. C:\Suspect. Now exclude that folder in the
Standard Shield, Customize, Advanced, Add, type (or copy and paste) C:\Suspect\* That will stop the standard shield scanning any file you put in that folder. You should now be able to export any file in the chest to this folder and upload it to VirusTotal without avast alerting.
So having created the suspect folder and excluded it restore your back-up copy to that folder, avast may alert when you try to do this so it might be best to pause the standard shield to do this.