That refrain is common enough that I even ended up reading my homeowners policy, and I didn't find anything in there that excluded unpermitted work, even if I did the work myself, even if I failed to follow code.
And if you think about it, such an exclusion would be crazy. The average homeowner is in no position to judge whether the work someone they hired does exceeds the scope of their license, or whether all the details of the work that need a permit get one, or whether work any work that doesn't get inspected is in fact done to code. Furthermore, the average homeowner isn't the original owner of their house, and is in no position to know what tomfoolery the prior owner might have done.
For those of you out there that are certain that DIY electrical work voids your homeowners insurance, can you prove any evidence to support your claim? A local news story about a guy whose homeowners insurance denied him because he DIY'ed something in good faith? A FAQ page from an insurer that says that DIY'ing something voids the insurance? Anything other than a vague reference to there being a lot of words in your policy and it gets interpreted through the lens of an enormous body of caselaw and the insurance company can afford more lawyers than you?
Insurance regularly pays out when people do stupid stuff and cause a loss to their own insured property, as long as they weren't trying to do damage. Doing stupid stuff that happens to involve electricity doesn't seem any different to me. And frankly, building your own L2 charger after much research and thinking about the safety aspects seems like a far cry from a "here hold my beer" moment.