what would be the reason for seeing question marks in my ARP table?

Though I can't find a reference, by guess would be the ? indicate the entry was created "on the fly" as opposed to entering the linkage when the device is added to the network.

If you manage a network, you would create an arp table to restrict which devices are allowed. Nobody is doing that on your Mac. It just uses the broadcast message to find devices and allows it and maps the MAC to the IP address.

And, doing more searching, https://stackoverflow.com/questions/4317049/arp-command-what-does-it-all-mean-and-getting-a-wireless-router-mac-address

This seems to be saying the same thing as I stated.

Your Mac isn't running a "true" DNS, so there is no truth data to compare the MAC and determine if it is really the host it claims to be. So, technically, it could be something spoofing an address, but unless you map out your entire network, the ? means nothing.

That website is just spreading FUD much like the OK sign is a "white power" symbol. They picked a normal output of the command and falsely assigned it a malicious response.

I cannot answer your question, but Linc Davis (supposedly the author of the script) has not been active here for quite a number of years - he just suddenly stopped being a contributor. It has been at least 5 - 8 years which - to me - would mean that most everything no longer applies given we are now 5 - 7 Mac OS versions later. Especially when one considers that we are now on Big Sur which has changed just about everything including the file system and the fact that you can no longer access your OS or system since they are now "sequestered" in a read only volume - neither you nor anyone or anything else has write access.

And yes, Linc was quite vocal about etrecheck. And, no, it is not malware.

And, important add on: I do not find any website that does not identify itself as to owner of site/company/ blogger/whatever with absolutely no way to contact him/her/them as trustworthy or, actually, I would not consider it worth looking at.

Ha, I just decided to get rid of cookies as I am sure that site planted a few (I do this anyway about once a week except for my financial sites, etc. where it'd be a pain to have to enter login credentials).