That's the contradiction in your whole proposal. You can't both use a centralized (or multiple) website(s) and claim the project is decentralized. Also you can't not download the whole blockchain if you want decentralization.
You see this is a flaw in blockchain technology, it is not this magical tool that can solve every problem. It works fine for a payment system because of the incentive to run a full node outweighs the problems associated with downloading and verifying the whole chain but it won't work for anything else.
You are not wrong but you are missing the important part. Sure the front end will be centralized but the back end (most important part) will be decentralized. This idea is not about being able to claim decentralized (misleading thread title) but it is about everyone being able to take part in the discussion.
Think of this more of a way for the future of Twitter to take the political and moral part out of censoring tweets. If all tweets were created by a transaction on a blockchain, anyone would be able to say what they want. Twitter may only choose to show the ones that abide by their terms of service but there would be another website which may show all tweets.