Note the use of the term relatively sudden.
I guess the question depends on your definition of 'explosion'.
My point is the creation representation of the Cambrian radiation as happening all at once is unfounded.
There are roughly 36 phyla give or take a couple depending on who is doing the classification. Of those 36, a large segment are worm-like with small variations between each phyla (including chordata). A smaller but still significant group of phyla are sponge-like. If the average person were to view the original representatives of all the phyla and compare them to the modern representatives of those same phyla, they would come to the conclusion that the average variation during the Cambrian looks to be less than the variation between modern and archaic forms of a specific phylum.
Only if you compare the typical modern forms of various phyla does the variation look highly significant. This appears to be why most people are shocked by the rapidity of the development of current phyla, they immediately compare modern chordata (fish, mammals, reptiles, birds, etc.), insects, sponges and worms with their large apparent differences, rather than the fossils with their much smaller differences.
That by itself is an important point. (i.e. different life forms are not as easy to discretely categorize as one might like)
I know you realize this, but just thought I'd also point out (to anyone else reading) that there are 36 or so animal phyla; fungi, plants, protists, prokaryotes, archaea, etc. are not included in this total. Another important point, is that in addition to taking tens of millions of years, the Cambrian explosion specifically affected macroscopic, animal life (a relatively small niche on the genetic 'tree') - it is only a universal explosion of diversity from the 'prejudiced' point of view that macroscopic animals are somehow a "directed goal" of evolutionary development (an understandable 'prejudice', considering that we fall in that category, biologically speaking).