There is a split here between law and practicality for many people I guess. As a matter of law for US Citizens it doesn't matter whether there is an "exemption" or not: a US Citizen may not be denied entry at a land port of entry, period. Their property can potentially be taken but only with legal process and it can't be kept indefinitely without violation of law. If they're wanted for a crime they can of course be arrested from which the normal legal process within the US plays out, but all the normal requirements are there too. They can be asked additional questions and put through more inspection, but with citizenship established that's it. So if someone simply refuses to answer any questions or unlock their phone and there is no further reasonable cause there is nothing the CBP can legally do to keep them out.
But as a practical matter most people don't want to spend an extra hour or hours or even extra minutes going through a more detailed search for contraband or whatever else. Most don't want to, aren't ready for and/or can't afford having electronic devices held for days/weeks before getting them back. A lot of people simply don't know their rights. So without an explicit exemption a lot of Americans undoubtedly would submit "voluntarily".
So the ACLU isn't wrong (and their actual page is appropriately more nuanced [0]): Americans aren't "subject" in the legal sense to this, or to any other questions beyond what's needed to establish citizenship. Having done so they may politely insist on entry and refuse anything else, demand to see a supervisor if an agent persists in unconstitutional questioning, submit any property required while in response demanding receipts and pursuing complaints or legal action afterwards (or both), and at the end of the day the CBP must put up or shut up: let them through or arrest them, and for the latter will have to meet the legal standard and it'll all play out domestically. But at the same time the financial and other burdens this imposes are very real and serious.
Best practice would be to go as "clean" through the border as possible, preferably with a dedicated phone that only has minimum necessary travel and navigation data on it and nothing else, no logging into any personal accounts of any kind, no bookmarks or the like, and cheap enough to not care about losing it. Then one can just let border agents look through whatever as much as they'd like, or let them take it and just write it off. Not everyone can or knows to even consider that possibility though. And of course the vast majority never have a problem, so it's insurance against a "black swan event" for the average person (those who suspect they'll be subject to heightened scrutiny legally or not may already do this). It's valuable to note both the exact state of the law and when the practical effect is different.
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0: ACLU: "If you are a U.S. citizen, you need only answer questions establishing your identity and citizenship, although refusing to answer routine questions about the nature and purpose of your travel could result in delay and/or further inspection." Or later "U.S. citizens have the right to enter the United States, so if you are a U.S. citizen and the officers’ questions become intrusive, you can decline to answer those questions, but you should be aware that doing so may result in delay and/or further inspection".
So the ACLU does acknowledge a practical cost to insisting on your rights, they aren't merely blindly saying "not subject".