it’s worth noting that the piece’s concern is with a somewhat passive experience of commonlisp, i think. for example the focus is on dealing with ansi=>sbcl types out-of-the-box; if i’m not mistaken there are no type definitions.
this is a useful lens through which to glean an accurate portrait of a partiality of what’s possible with types in and around modern lispcraft, maybe, i think.
it’s difficult to think about hackability in writing bottom-up programming code without thinking about typing at or near runtime and/or/either typing either/or/and retyping some clos mod or mop ( or any-&-everything else ) during some runtime.
this is to say nothing of the conditions system modulo defining types or incorporation of domain specific, embedded or foreign languages via same modulus.