Now I'm curious where the original Apple logo came from…

(And now I know: designed by Ron Wayne, so not public domain at all; and I'd forgotten about the ribbons with the text)

https://en.wikipedia.org/wiki/Marketing_of_Apple_Inc.#/media...

Of course you can.

3M, Scotch, Crate & Barrel, Lufthansa and Jeep all have legitimate trademarked word marks that consist of nothing more than their name written in Helvetica.

And every single one of those is in the Public Domain, because they don't meet the Threshold of Originality for copyright purposes.

https://commons.wikimedia.org/wiki/File:3M_wordmark.svg

https://commons.wikimedia.org/wiki/File:Scotch-logo.svg

https://commons.wikimedia.org/wiki/File:Crate_and_barrel_log...

https://commons.wikimedia.org/wiki/File:Lufthansa_Logo_2018....

https://commons.wikimedia.org/wiki/File:Jeep_logo.svg

For more info:

https://en.wikipedia.org/wiki/Wordmark

https://commons.wikimedia.org/wiki/Commons:Threshold_of_orig...

Which is irrelevant to whether or not they are valid as trademarks

That's a word mark... it doesn't matter what font it's in because the mark relates to the words, not a specific design of the words in display...

The word mark registers the word as a trademark.

The typographical and design treatment of the word can have separate trademark protection.

That can be true whether or not a design mark is registered.

If you started a car company called ‘Beep’ and write its name in olive green Helvetica Black, Jeep might struggle to make a case about your name’s similarity to their word mark (beep is a different word and rhyming is not a crime), but they for sure could take you to court for stealing their trade dress.

So rather than make the point that the use in helvetica is captured in the word mark (it isn't) you instead add trade dress into this??

Not in the word mark registration, sure. But registration is not the whole of trademark law is it?

‘Word mark’ is also just a term of art in graphic design referring to a logo consisting of just text - another word for logotype. Should I have said logotype instead?

It's captured in the trade dress - that's all you had to say. You don't have to say its captured in the word mark, because it isn't!