Does the saying “physical access = game over” apply to smartphones, too?

I was surprised to read in the responses popular question that it’s considered nigh impossible to secure a computer system if intruders have physical access.

Does this apply to smartphones as well? Let’s suppose I have done the most I can to secure my phone on a software level (e.g. encrypted storage, restricted app permissions … whatever you consider “maximally secure”). Is physical access still game over?

This seems like an important issue, as there are many situations in most people’s daily lives where they leave their cell phone on their desk as they take a break or what have you.

Technical answers are welcome, but I would personally appreciate responses that are legible to someone without a background in information security.

This question is related, but it deals more with surveillance technologies that might be built into a smartphone, not the options available to a malignant individual with physical access.