A web browser allows access to all websites (broad reach). HN is just one website on the web (narrow reach). Pretty clear to me why this is a bad comparison, just interested to know why the OP thought otherwise.
We're talking about advertisements and the advertisement's only one one page. The scope of the actual advertisement is roughly the same as it is on HN. The fact that an advertisement in the default tab of a browser is likely to have more viewers is irrelevant.