I don't think that is true, in the way that it always wasn't: How would you be able to tell when it's done properly?

Think WordPress installations: Depending on how it's done you can either tell at a glance (probably ~90% of WP installations at some points in time) or you have no clue until you look at the html source.

Of course, when given the option to not do it properly is always alluring and then you can tell.

So you can tell for maybe 20% of websites that have been generated by LLMs over the last few months.