In my opinion, it looks very much like concrete/mortar and very similar to other photos I've seen of field applied dried mud/clay mixtures. Both of these types of materials were commonly applied in the field by axis forces in Italy (and other locations) according to first-person narratives. That is, German tank maintenance personnel, crewmembers, and commanders have described the use of these materials in their memoirs or in official unit histories.
IMHO, many historical authors over the years have mistakenly used the word "Zimmerit" as a generic term to describe ANY non-magnetic material applied to vehicles. While I have no way of knowing why this author has chosen to use the word here, recent other errors in the captioning of historical photos lead me to be leary of relying only on this evidence.
In any event, people will have to make up their own minds. If you choose to take the author's statements as completely true, it's Zimmerit. If you don't, it could be any hastily applied material. There's really no way to make a definitive determination one way or the other based on these photos alone.