For example, he can use the Marines, which are not covered by the statute.
But, even so, posse comitatus is to limit the use of the Army to enforce domestic policies. Defense of the border, on the other hand, is the first and primary purpose of the Army. I don't see how posse comitatus applies in this case at all. Why couldn't trump order the 1st Cav to defend our border? I don't see any legal limitation on that.
Even if there were such a limitation, Trump could just use (i) the commonly accepted Eisenhower exception, which allowed him to send federal troops to enforce the desegregation laws in Little Rock, Arkansas, or (ii) the effective but illegal Clinton drug exception which Clinton used to butcher that cult in Waco (the drug allegation was later determined to be a lie by the ATF, so there are easy ways to get around posse comitatus). Bottom line, when local authorities are unable (or even unwilling) to enforce the law, the military can be used (Eisenhower), or the military can be safely used when the targeted people are sufficiently marginalized (e.g., Japanese-Americans, and perhaps immigrants but that remains to be seen).