When you say how they can "legally" replicate it, let's first acknowledge that 99.99% of what we do, our work, is replicating something, reflecting, transforming, combining, sure, but mostly replicating things.
You think the similarity is notable, broadly. Fine, but let's break it down in legal sense. What kind of claim can you have here? Clearly, the icons are not copied, they're just similar. The layout is not unique either, millions of sites have one like it.
The round blue button and other unnecessary similarities seem to rub it on the nose "hey I'm Twitter" but I don't think it's illegal, unless Twitter has a design patent for their UI.
So if it's not copyright and not patent, what would it be? Not trademark, either, as it says "post" not "tweet".
Of course we can say "they have retweets" and so on. But those were concepts invented not by Twitter but their users, in the open-source, no-ownership way. Twitter can't claim to say they invented post sharing. They just can't.
Their only moat is the network effects of their userbase. I see no technical or design moat.
Keep in mind Android copied iOS wholesale. It copied iPhone wholesale. All the things. All of them. Apple tries to sue about some trivial aspect they had a design patent on, and lost.
Sometimes you see they copied, and it's blatant, but upon closer look, there's nothing illegal. And maybe it's for the best. Nowadays Android copies iOS, iOS copies Android. You could say it's a beneficial relationship of copying.
But if Twitter dies because someone copied it... it means Twitter had nothing more to contribute. Apple has a lot more to contribute, this is why they're doing fine, despite everyone copying them.