The notorious "A device using your internet connection may be infected with malware" is not entirely true, which the link you provided shows VM admitting false positives. If the letter or email has not dates you can just dismiss it. It's always easy to say "We’ve been notified by a third party (whoever they are) that malicious traffic has been detected coming from a device using your internet connection" yet VM cannot say what device or the type of malware. Most annoying, especially so when they lock your main or email account, resulting in you having to change email address passwords, and is that lies the truth. The world knows VM passwords are dire, so I put it to VM that it's a hacker trying to gain access into peoples account, whether is be the main or email account.
However, I would recommend you scan all your systems and if you don't find anything you could well be clean. I would recommend you download Malwarebytes (it's free for a period) as that market leader by miles on malware detection and prevention. Under security you can almost select everything bar Delay real time protection. Under Advance Settings, Java Protection select all bar the last item. Then select the next tab titled Penetration Testing (Note: this can produce the odd false positive, yet you can create an exception for the item in question if you know for sure it's okay, but you can notify Malwarebytes Forum where if it's a false positive they can add it to the list of known items.) I would also recommend you purchase a full licence as it will prevent malware from infecting your PC/Mac before Norton does.
"I have various gadgets linked to my wi-fi such as my EV charger and indeed my car and I've not got a clue how I'd scan them for malware," and in that lies the crux, you cannot. You need an addition piece of kit to protect IoT devices and Bitdefender Box springs to mind, as the firewall setting for such items is locked down and I believe (you need to check) that you must agree that items in yours and not something elsewhere hooked into your network without your knowledge. MAC filtering can help towards that goal.
Panic not, you will not be cut off unless VM can prove the malware is coming from and what's being distributed. VM is not a bad company, I have endured 20+ years of only a handful of problems but it's when it goes wrong through poor investment (almost all other ISPs have caught up with VM now) it now seems to be falling apart. VM needs to force people into having longer passwords with special and ambiguous characters. Users to start using a password manager so they only have to remember the one password, so when you login you unlock your password manager as it can enter usernames and passwords in a split second. Also, if you want to be more secure, check all online shopping, social media, etc., accounts for 2FA authentication that resides on a smartphone or an external key, or for guaranteed safety a Yubico Key (actually you purchase 2 keys,) as they make the keys for phones and tablets as well, so you set up the first key, duplicate via Yubico website after registering the keys and clone the second key. Two key are so one key that stays with you and the other key kept in a very safe location in case you lose the first, as without the key you cannot gain access to your accounts yet prevents hackers 100% as no hacker can in without your key.