The <i> tag in modern HTML does not mean "italics", though the examples given to illustrate its new semantics are all things conventionally set in italics.

"The i element represents a span of text in an alternate voice or mood, or otherwise offset from the normal prose in a manner indicating a different quality of text, such as a taxonomic designation, a technical term, an idiomatic phrase from another language, transliteration, a thought, or a ship name in Western texts." [0]

Its use for text-as-icon is arguably, despite obviously not being a use classically set in italics, within the defined semantics of the <i> tag.

EDIT: Though FAs use isn't actually text-as-icon, it uses empty tag as an icon. That's not consistent with the semantics of the tag.

[0] https://html.spec.whatwg.org/multipage/semantics.html#the-i-...

<i> is not the "Italics tag":

In earlier versions of the HTML specification, the <i> tag was merely a presentational element used to display text in italics, much like the <b> tag was used to display text in bold letters. This is no longer true, as these tags now define semantics rather than typographic appearance.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/i

You can use whatever <element> you want with FA. It's the class names that are important.