Thank you for this. There is occasionally disagreement about what "Common Lisp" even means, and the spec is often cited, but as far as all of my posts, library work, and application work are concerned, Common Lisp means "the current reality of the major compilers as implemented in 2025". This is a descriptive / bottom-up definition, and as an active author of software it is the one I'm more concerned with. For instance, `:local-nicknames` have been essentially universally implemented among the compilers, despite not being part of the spec. To me, this makes that feature "part of Common Lisp", especially since basically all CL software written today assumes its availability.
You're right to point out too that the post is somewhat SBCL-centric - this too reflects a descriptive reality that most new CL software is written with SBCL in mind first. Despite that I'd always encourage library authors to write as compatible code as possible, since it's really not that hard, and other compilers absolutely have value (I use several).
Every programming language has a practical definition: it is the intersection of the sets of features that are accepted by the various relevant production compilers and interpreted identically enough to be portable to all of them.
Formal language definitions, standards, and books are great, but you can't compile with them. Abstract language specs that don't have reference implementations or conformance test suites are not particularly useful to either implementors or users.