gk7ncklxlts99w1 "Most secure OS in the word" is always a tricky statement, it probably is, but the user shouldn't believe that GrapheneOS on a recent Google Pixel is a magical bulletproof fortress, that's not how it works.
The GrapheneOS project account or a project developer are the best members to answer this question, but to give my pinion on the spot, I can say that many mobile devices sold “for security and privacy” are overpriced scams and target a category of customers who think that “high prices + secrets = data security and privacy”, these companies often use a spy agent movie atmosphere in their marketing.
I've never heard of Solarin, Sirin, K-iPhone and Bittium and I wouldn't trust his devices.
Blackberry hasn't been making security oriented phones for years. They used to use a microkernel-based architecture, which was good.
Murena and Purism, bring neither security nor privacy; on the contrary, they massively reduce it as far as I know.
From what I found out in a discussion with a member of the Molly project (hardened fork of Signal), apparently, the only phone to compete, or to have competed, with GrapheneOS on a Google Pixel is the Cryptophone from GSMK but they only sell to companies and are not suitable for use in everyday life, you can't even buy one as an ordinary buyer. I know very little about the Cryptophone so don't ask me questions to find out why and please note that I'm not asserting anything.
Some of these phones, sold at too high a price for their “security”, use operating systems that are in fact modified forks of GrapheneOS.