Here are 2 screenshots from Chrome on Android displaying the article. The one in the left is how your article looks by default. The readable version on the right is how it appears when you accept Chrome's offer to show a "simplified version".

https://photos.app.goo.gl/ntvIcKSDTOfdkwAi1

> The one in the left is how your article looks by default.

I'm not the author of the article.

The problem might be due to antialiasing applied to the wrong font size. On my desktop the page is easy readable, but some letters have their vertical lines appear as two grey lines instead of a single black one or a mix of the two, in other words although it's 100% black text, the antialiasing renders some parts of it like 50% black to make it appear smoother. On a phone it might be even worse. Changing the font size can help.

Not an expert in anything about web publishing or similar stuff, so forgive if I used the wrong terms.