How does a non-segwit full-node validate a SegWit input
0 and ab68025513c3dbd2f7b92a94e0581f5d50f654e7 in stack-based bitcoin
  script does not return true


They do return true. Here is some pseudocode:

push ( 0 );
push ( ab68025513c3dbd2f7b92a94e0581f5d50f654e7 )
return ( pop ( ) != 0 ); // evaluates to true