Not so.
They are limited by convention and policy.
There is no law that stops the US from using the military to prevent unauthorized entry.
==================================================
The Posse Comitatus Act pertains to the use of the military against citizens in matters of law enforcement. Not against foreigners coming from another nation.
The Posse Comitatus Act and Related Matters
https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/p/posse-comitatus-act-and-related-matters-a-sketch.html
When the Act Does Not Apply
"Constitutional Exceptions: The Posse Comitatus Act does not apply "in cases and under circumstances expressly authorized by the Constitution," 18 U.S.C. § 1385."
...
Military Coverage
Navy and Marines. The Posse Comitatus Act proscribes use of the Army or the Air Force to execute the law. It says nothing about the Navy, the Marine Corps, the Coast Guard, or the National Guard. The courts have generally held that the Posse Comitatus Act by itself does not apply to the Navy or the Marine Corps. They maintain, however, that those forces are covered by similarly confining administrative and legislative supplements, which appear in the Department of Defense (DoD) Directive." (10 U.S.C. § 375)
https://www.history.navy.mil/research/library/online-reading-room/title-list-alphabetically/p/posse-comitatus-act-and-related-matters-a-sketch.html#geography
10 U.S.C. § 375
"Sec. 375. Restriction on direct participation by military personnel The Secretary of Defense shall prescribe such regulations as may be necessary to ensure that any activity (including the provision of any equipment or facility or the assignment or detail of any personnel) under this chapter does not include or permit direct participation by a member of the Army, Navy, Air Force, or Marine Corps in a search, seizure, arrest, or other similar activity unl ess participation in such activity by such member is otherwise authorized by law."
http://policy.defense.gov/portals/11/Documents/hdasa/references/10_USC_375.pdf
Here, we see that the Secretary of Defense is the one who prescribes the regulations that limit the Navy (& Marines), not the PCA.
Speaking of "cases and under circumstances expressly authorized by the Constitution"
invasion noun
1. an act or instance of invading or entering as an enemy, especially by an army.
2. the entrance or advent of anything troublesome or harmful, as disease.
3. entrance as if to take possession or overrun:
4. infringement by intrusion.
Foreign nationals streaming into our country who "overrun" our border, or "infringe" by "intrusion" (to thrust or bring in without invitation, permission, or welcome) are by the very definition of "invasion", and the fact that they are not citizens of the U.S., subject to the operations of any branch of the military because the Constitution overrides the PCA.
Hat tip to Travis McGee for the graphic.