That is objectively untrue.
You do not need any houserules or homebrew whatsoever to make 5e deadly. You literally just build deadly adventures. You control how much damage things do, what traits monsters have, what sort of effects traps have, what enviromental hazards do, etc. 5e presents all of these things in a way that is quite easy to understand, and quite robustly balanced, so that they will usually do what you expect them to do.
The only real differences are presentation (ie, the published APs are quite easy) and the fact that in 4e and 5e the DM is much less likely to be surprised by how deadly or easy a challenge is.
To be clear, I'm not even talking about the DMG optional rules, which are absolutely official parts of 5e, they're just optional. You don't need to make healing take longer, or change what is recovered with a given type of rest. You only need those tools, which again are part of the official game and can be found in the core book that is made to help DMs run the game, if you want to challenge the players in exactly the same way as in older editions.
If you're willing to learn a new way, you don't need them at all.