The short answer is because his proposed "solution" isn't solving the fundamental issue of fairness in public communication, nor does it guarantee any meaningful improvement thereof. Case in point: Elon is perfect, makes Twitter the best, then dies the next day and we're back to square one because the will of one man is not an institution (explicitly defined in this context as "surviving those who man it").

The long, IMHO, is that eventually the only solution is a truly decentralized system, much like DNS, that no actor alone can enable nor stop as long as other actors refuse to "fork" the namespace for a more restricted one.

That's what "freeing the code" ultimately means, and that comes down to a protocol at a basic level. Anyone (company, individual…) being free to implement whatever interface they want to it.

Then we'd likely move to observing and participating in this unimpeded common "mother / raw / source" stream of messages, through what I imagine would look like more-or-less-biased portals of sorts, culturally flavored aggregators, but ultimately unable to lie about true raw numbers.

Until we all agree about reality, there's no discussing it.
That's why we need dumb pipes, neutral transport, much like the air is dumb in transmitting sight and sound.
That's why we make protocols (as in RFC's), and every single internet-facing machine holds its little piece of HTTP land, of IRC land, tomorrow of <some Twitter-like land> (I know, it's Mastodon, but it's also not in reality as we speak. Ask your sales people about adoption rates and network effects).

Thus follows, IMHO, the answer to your question:
Elon claims a lot of things but none guarantee such a free and fair space for communication that de facto puts him out of the loop as a necessary gatekeeper (notice how Mr. Musk's moral fairness or even good intents, as a "benevolent dictator" is Montesquieu's words IIRC, is actually irrelevant to the core question).
Elon does not suggest that he would "protocolize" Twitter's operations such that the company would act as a central but ultimately _optional_ hub for the sake of a mission —and literally operation— bigger than itself: free and fair human communication (by which point who cares about their code since everyone else can check the raw data stream in countless replicas and redundant streams over the web).
Twitter (Inc.) as the steward and creator of this protocol would actually become valuable over the very long term (think: E/P ratio, employer prestige, etc) because hosting a fair and free human communication is one hell of an important job that few institutions —ever in history, private or public— would ethically be able to manage. But it's now become military-grade or hospital-grade important to a free society. As I was saying in another thread, it's about time we manage some deontological framework for the field of information technology and its practice at large.

[PS: Not that I hold my breath for one second that Twitter as we know it is in any way willing let alone able to undertake such an ethical mission, nor do I dream for one minute that Elon would buy Twitter instead of going from scratch, were the above goals his. The true disruption will likely be menial at first and come from left of field.]