It's all in how you say it.
This is true in more ways than one. Trump is intentionally creating the image of a serious border problem when one no longer exists. Since Obama's administration greatly stepped up deportations the flow of illegals has dwindled to a trickle. Further measures have only political value. From a practical standpoint they are cost ineffective. Trump has to carry on with his subterfuge however. He made illegal immigration the center piece of his campaign. But, as we have all learned since his election, his knowledge of immigration law, U.S. law in general, the Constitution, Government, and World Affairs is minimal, while his lies and made on the fly facts are endless.
You guys correctly noted posse commitatus would seemingly bar Trump,
in the absence of a crisis, from legally ordering regular military troops to protect the border, augmenting and overlapping the duties of regular border patrol personnel . Mattis is against it. Mattis knows the law. It is doubtful Trump discussed this questionable deployment before blurting it out. If he ignores good advice, and orders Mattis to deploy troops along the border, no doubt troops will be deployed. There would be zero chance that this present Congress would step in and stop it.
We have deployed military troops domestically before, but it was always under dire circumstances that exceptions were made. For example, during the civil rights movement, when Bobby Kennedy ordered Ross Barnett to admit James Meredith to Ole Miss, U.S. Marshals along with thousands of Federalized National Guard troops, making them U.S. troops, were deployed to the Ole Miss Campus and surroundings.
Exceptions to posse comitatus are few and far between. Trumps desire to play to his clueless base clearly does not satisfy the accepted criterion needed to justify an exception. And certainly the trickle of illegals still surviving the crossing of the U.S. border with Mexico can not be truthfully described as an invasion.