Who in his right mind would upload confidential files to any sites without even knowing who manage it... Do you send your taxes reports to some unknown dude offering you free checking? I guess no.
But shouldn't these malware scanning sites also make clear that uploaded data is published and available to other people? But yes, even if this wasn't the case, why on earth would you trust these malware scanning sites with your documents.
Misuse of Alphabet’s Virus Scanner is Exposing Sensitive Files October 21, 2019 https://finance.yahoo.com/news/misuse-alphabet-virus-scanner-exposing-230000762.html