Northeast of El Campo, Texas
They did mean it.
AMTRACs and infantry—the bulk of 2nd Marine Division—moved slower than LAVs. And, though the Marine Corps had LAVs aplently, they were to a large extent constrained by the speed of their slowest movers; in this case LPCs, or Leather Personnel Carriers.
Thus there was plenty of warning that the Guard was serious and that no more warning shots would be fired.
During the night, small parties of Marines went forward to recon the west bank of the river from the east bank. One and all they came back with the report, "Too hard." That is to say, they all came back with the report except for one patrol that was caught trying to cross the river. This patrol did not come back at all. In its way, this confirmed what the others had said.