The only thing the 2 have in common is the use of HTML attributes for functionality. Completely different on every other axis that matters.
In another comment, you mentioned State Management. If this is on your mind then you are using htmx wrong. You should not be managing any client side state with htmx. State is on the server or in your database. Interactions on the client should immediately reflect the updated server state. If you have separate state on the client that needs to be managed, you are going to have a bad time regardless of framework.
Yeah, because clients never need to keep track of state like which block is expanded or which tab is selected or that the previous page was or what the user is currently typing...
Say you have a huge Google Ads budget burning up on those landing pages. Make one version with React and one with HTMX. Open Lighthouse and see performance difference. First page renders are much slower with React. Page load is slower and this makes a huge difference in mainland USA where people are still on 3G speeds on their budget phones.
"High traffic" might be the simplified view here and it took me a few page memos to explain it to my lead but the gist is that React is slower, takes longer to load and most of what it is used for is easily done with plain HTML and sprinkles of JS as needed. Unless you're streaming sound or video like you do on Facebook pages it's really not the right tool for the job in a majority of cases.
The gist of it continuously seems to be that people who think HTMX Great, know very little about good frontend engineering.
But I will answer your main questions since your comment seems to be in good faith.
Just like angular 1 choked on complex, high-frequency UIs, HTMX is going to follow the same faith. Github uses a similar approach to HTMX and you will find that even for such a simple system, their notification indicator elements on the same page are often out of sync.
The reasons for this is that State Management is hard, which is why the reconciliation loop of React-like frameworks and shadow-dom does away with pushing complex state management to user application and handles it systematically.
Of course, this doesn't mean htmx is useless, for a specific case of web apps, it is good enough, but the problem is that often times, you start with "specific cases" and your application grows overtime.
So when you consider that and the cost of doing htmx vs React or any react-like framework, picking htmx is only reasonable if it is the only option you're comfortable with and rarely on merit.