Well, once you have been told to unlock the device, you're already in a legally binding process. The phone is at this point evidence. It was not evidence before. It was not evidence a month ago. It really is just that simple.
Now, they could view prior wipe as suspicious, but as a US citizen they cannot prevent entry. And they may be able to seize your phone(suspicious!). Which is why simply stating the truth politely "I believe in privacy, and loath government poking into the private affairs of citizens" might help down the road if you want to sue. Might.
Border guards protect the realm, after all, and have wide latitude.
From my side, my truthful argument for wipe has always been that all of my buisness clients, emails, data might be on my phone. I have a duty to protect their privacy.
Making reasonable statements takes the edge off of 'suspicious', and the more people who wipe? The less suspicious it becomes.
The biggest thibg anyone could do, is make 100% restorable backups for non-rooted Android a thing. It's doable, but a PITA right now. Make it one-click, perfect, reliable, and more will do it.
And then it isn't unusual, it's normal, and the suspicious elements vanishes.
Of course, as Google is mired in asshattery lately, I'd expect any attempts to protect us all, such as ASOP patches or bug reports, would be fought against and ignored. Helping the world, protecting travellers, political dissidents, not on their radar.
They even fight such things.
Because in this day and age, Google does not have your back. Instead, they shove knives there.