Good. <h1> tag should be <h1> no matter where it is. It shouldn't change; at least not by default. Even better that this change back to the old way will fix a problem for accessibility for screen readers.
The article seemed to repeatedly suggest that any <h1> without a font size set would be a warning, but reading more closely I'm pretty sure it's only talking about <h1> nested within <section> etc. (So I believe Lighthouse and Firefox are both already doing what you describe in the last sentence.)
I still think even one within a section (or even div like nested sections) shouldn't be a warning... but I hadn't heard of <section> until today (seemingly it's been in browsers for decades, I just hadn't seen it USED anywhere) so I'm not sure if that implies something other than a logical structural block. From the context it seemed like a different type of div or span.
Sections are a very good idea for a tag that embedded a document in a lower hierarchy inside your document. The way that it should be from the beginning instead of having the global h1, h2... and hacking a structure out of them.
But then screen readers universally decided that no, instead of using sections to navigate a page, they will just ignore the spec and flatten everything into the top of the hierarchy. So nobody uses them.
(And yes, it shouldn't be a warning to use an h1 in a section with the default style either. They are just making a non-backwards-compatible change to the default style, fucking own it and have the old pages change.)
"decades" I had to look it up, it's been 15 years apparently, with HTML5 being 17 years old... I quipped to my colleague that I'm still stuck at XHTML. Mind you, that's also because in 2008 we still had to support Internet Explorer 6.
I'd love the article to clarify this, because it stuck out to me as well. But as you pointed out, I think that's what they meant: the lighthouse warning is called "H1UserAgentFontSizeInSection"
