If you are thinking to build a very basic website, and you don’t mind working 100% within the design you purchased, then there’s no reason to learn HTML. HTML is not a true programming language, so it doesn’t result in new ways of thinking that allow you to view the world with an enhanced perspective. For most of the people, knowing HTML is a lot like knowing how to change your own oil, or cook a good meal. There’s value on it, but it probably isn’t necessary.
On another way, if you want a little bit of understanding about your website, it’s a nice skill to have. It’ll let you troubleshoot basic problems, and it’ll let you avoid paying someone every time you need a change that the author of the theme didn’t provide. Just because I take my car to the mechanic when it needs serious work, that doesn’t mean I don’t want to know how to change my brake pads or swap a spark plug.
In my case, I am a mechanic. I design web experiences for a living and I’m happy to let you pay me for it. Even as a professional, I enjoy clients who understand a bit about my craft. It helps me to feel valued and lets us have much more productive discussions. It gives me an extra measure of respect for the client as well.
Knowing HTML as a layperson isn’t necessary, but it is still valuable.
HTML is just the first step into a much larger world. There are lots of questions about it on Quora, do some searching. HTML is the most basic markup. It’s a document full of tags that we target with other languages in order to display and manipulate a design.
So why don’t we all just use Squarespace or Wix and call it a day? Setting aside the question of who would design anything if nobody cared for design, let’s address the idea that designers aren’t necessary to the average person.
Why should I learn how to write well when there are lots of good authors on Amazon?
Why should I eat at a restaurant when I can buy a frozen meal at the market and cook it at home?
For that matter, why bother learning to cook?
Why do people pay for music when it’s already available on the radio?
Why would anyone make a movie when there is so much to see on television?
Why would I want to own a dog when the city has a petting zoo?
Why would I buy a computer if there are literally seven of them at the library for free?
It’s all the same question.
If you aim to be a valuable creative professional, you will have no shortage of work unless you learn just one thing (basic markup) and stop. That won’t get you a job anywhere.
People like to be unique in many, but not all cultures. Even in collective cultures, quality matters to the group. Not everybody in the world wants to drive a Toyota Corolla or a Honda Civic. Your friend is suggesting that, when it comes to websites, they do.