Question: Why is direct assignment to list element not efficient?

I was browsing this Maple book, and read this below.  This contradicts everything I expected about direct assignment to an element of a list, which I view as an array and should have O(1) cost to modify any one element in it.

So, clearly there is something I don't understand about Maple here or I am not reading this correctly.  Here is the bit from the book I scanned. This is from Computing with Maple, by Wright, page 61, ISBN 1584882360

So is one really supposed to write  

subsop(index=value,L) 

instead of the more normal and natural

L[index]=value

For making changes to list L?

I also do not understand the book when it says "only allowed on small lists anyway". Allowed by whom? Will one get an error if they assign to an index on large List?

I am confused by all of this, and that is why I am asking for clarification.