When trying to compile your example (SimpleIntegerList.pde) in Arduino IDE 1.8.16 with a Nano RP2040 as target I get the following error:

/Users/(...)/Dev/arduino/sketch_sep22b/sketch_sep22b.ino: In function 'void setup()':
sketch_sep22b:26:10: error: 'class LinkedList' has no member named 'add'
myList.add(n);
^~~
sketch_sep22b:27:10: error: 'class LinkedList' has no member named 'add'
myList.add(0);
^~~
sketch_sep22b:28:10: error: 'class LinkedList' has no member named 'add'
myList.add(l);
^~~
sketch_sep22b:29:10: error: 'class LinkedList' has no member named 'add'
myList.add(17);
^~~
sketch_sep22b:30:10: error: 'class LinkedList' has no member named 'add'
myList.add(k);
^~~
sketch_sep22b:31:10: error: 'class LinkedList' has no member named 'add'
myList.add(m);
^~~
/Users/adias/Dev/arduino/sketch_sep22b/sketch_sep22b.ino: In function 'void loop()':
sketch_sep22b:36:25: error: 'class LinkedList' has no member named 'size'
int listSize = myList.size();
^~~~
sketch_sep22b:46:22: error: 'class LinkedList' has no member named 'get'
int val = myList.get(h);
^~~
exit status 1
'class LinkedList' has no member named 'add'

Any ideas of what I am doing wrong?