Everyone repeats this, but is there any information that confirms it?

Yes, it started as a DLC for Hollow Knight - but the devs have known for the past 5 years that it would be a standalone sequel. Is there any evidence that they designed it as "extended content for people who had already beaten the secret harder stuff", rather than approached it as a game on its own right?

A lot of people coming to Silksong having played Hollow Knight seems to be saying this, but also seem to have forgotten that pretty much exactly the same things were said about Hollow Knight when it first came out over eight years ago.

Almost like the collective memory of Hollow Knight's difficulty has dulled over time as people have, over the eight years, dare I say it, git good...

Are there things that are measurably more difficult? Perhaps. Common enemies can now do two masks worth of damage which before was relegated to boss specials, so environments feel more dangerous. This has always been a significant part of the early game, as an extra mask in Hollow Knight was significant enough to keep you safer from regular enemies, but in Silksong once you get hold of an extra mask you go from being able to be killed in three hits to...being able to be killed in three hits.

So I think there are things that make it feel tougher. At the same time though, all the same things were said before about the difficulty, about not knowing where to go (Silksong gives even fewer clues I'd say), but people persisted through areas, learnt boss patterns, and eventually just learnt the game up until P5.

And in a few years the collective memory of Silksong's early game difficulty will have gone as people adapted to it.