Mexico does not enforce its borders
These people crossed two of them to get here
Mexico looks the other way at Central Americans who are ferried through via corrupt smugglers and their assurances that these migrants will not be staying long in Mexico. All necessary palms are appropriately greased.
A Central American, on their own, who crosses into Mexico will not be so welcomed nor have the authorities look the other way.
This is the difference.
Oh, and Mexico doesn't really want these people back. Mexicans are also in the process of "Making Mexico Great Again" through exporting their illiterate, unskilled Mestizo underclass to us.