Because you have to pay to get the actual content. The title says “How integers and floats work.”But it isn’t a post about how integers and floating point precision numbers are represented in binary. Rather it’s a post announcing an illustrated guide to ints and floats, which you have to pay for.

> Curious as to why you thought that this was an ad?

Because it is clearly an ad? It explains what will you get and at what price. It does not explain "how Integers and Floats work". They want you to buy the zine, which is what they are advertising.

> Julia Evans has written a bunch of such zines in the past as well and they are quite informative

Good for them. Why do you think this makes it not an ad?