The truth may be somewhere in the middle.
There is a popular school of thought, as The Post's Carol D. Leonnig and Robert Costa noted in the piece, that Mueller may not even view charging the president with crimes as a potential outcome of the investigation. It has to do with an opinion written by the Justice Department’s Office of Legal Counsel in 1973 (read: Nixon, Richard) that was affirmed in 2000 (read: Clinton, Bill).
Here's what that Office of Legal Counsel said in 2000, from then-Assistant Attorney General Randolph Moss:
In 1973, the Department of Justice concluded that the indictment and criminal prosecution of a sitting President would unduly interfere with the ability of the executive branch to perform its constitutionally assigned duties, and would thus violate the constitutional separation of powers. No court has addressed this question directly, but the judicial precedents that bear on the continuing validity of our constitutional analysis are consistent with both the analytic approach taken and the conclusions reached. Our view remains that a sitting President is constitutionally immune from indictment and criminal prosecution.
That second sentence is key: “No court has addressed this question directly.” That means Mueller isn't bound by this opinion, which is after all merely an opinion.
But he is a longtime creature of the Justice Department who may decline to step outside the bounds of what the Justice Department has previously recognized as its authority. Going outside those bounds would also potentially invite allegations of overreach — of which Trump and his defenders have already accused Mueller's investigation — and could complicate any political resolution (i.e. impeachment). In other words, Mueller has plenty of reason not to try to charge Trump with crimes, even if the evidence would lead him to charge basically anybody else.
Which brings us back to this message sent to Trump's lawyers. What if Mueller is saying Trump isn't a criminal target of the probe because he doesn't think Trump can be a criminal target of the probe?
This could be a significant moment, suggesting Mueller views criminal charges against Trump as being off-limits. And that would surely disappoint Trump's critics. But if that is what Mueller is saying, it also means declaring that Trump isn't a criminal target says basically nothing about the evidence at hand. It would mean Mueller could have the most damning information about collusion, obstruction of justice and anything else, and he would technically be telling Trump's lawyers the truth when he says Trump isn't a criminal target. It also wouldn't foreclose impeachment.
Notably, Leonnig and Costa also report that Mueller's team has indicated it might roll out its findings in a series of reports. Mueller, if he sees what would otherwise constitute criminal activity involving Trump, could simply put this information into one or more of those reports and leave it to Congress to decide what to do. That may not be as edifying to Democrats as it could be — and Republicans could ostensibly block any effort to impeach Trump and remove him from office — but the point is that this doesn't necessarily mean Mueller's evidence is weak.
“The ‘subject’ status may inform a special counsel report to Congress more than a prosecutorial decision,” said Jack Sharman, a former special counsel in the Whitewater investigation into Clinton, “especially since the weight of authority — although not unanimous authority — is that a sitting president may not be indicted. "
It's also worth emphasizing in all of this that Mueller didn't do this just because he wanted to. Targets of investigations generally should be informed that they are targets, according to Justice Department protocol. So this likely isn't Mueller playing games by luring Trump into a false sense of security; it's Mueller doing what he is supposed to do.
In the end, this could mean any of a number of things, but neither side should take this as foreshadowing of any specific or likely outcome. It's also entirely possible Mueller does think he can criminally charge Trump, but honestly doesn't view him as a target at this juncture. (And that could always change.)
Like almost everything in this investigation, only a handful of people know — and they're not talking.