When they say “can’t be blocked” I presume they mean “can’t be blocked with the block function in X/Twitter”. They also say it can’t be liked or retweeted.
So far ads have been treated as sort-of regular posts that are just shown according to the ad rules rather than because they belong in the timeline under normal criteria, and you could like, retweet and block them just like any other post.
So this is basically them treating ads as a fully separate thing rather than just a different post type.
Though the article suggests they’ll still try to make them look mostly like posts, except without showing a handle etc. though, which is extra scummy