But yes it is meant to be special (like read) and also has its own special syntax for formatting. However this isn't anything weird as the compiler was already implementing all other standard procedures and functions as part of the language itself anyway - there isn't a distinction between 'language' and 'runtime library' like you'd see in C for example.
It is better to think read/write as akin to 'while', 'if', etc than as regular procedures. Making your own 'if' might be neat, but not something you'd see often outside of Lisp :-P
It was not addressed in his Pascal versions because he moved on from Pascal, not because he didn't recognise that Pascal was too limited for many use-cases.
http://pascal-central.com/docs/pascal1973.pdf
It doesn't mention Writeln anywhere. In section 13 (page 39) "Input and Output" it only mentions read and write for text input/output and at the end of it mentions that the end of each line must be indicated with the EOL characer. A few pages later (page 44) there is a table of standard identifiers and there is no mention of writeln either (but there is of write).
Confusingly enough though it does describe readln and writeln in an attached revised report (mentioning that the reason for their addition is that they can't rely on an EOL character being available) but this is not the text they cite as the standard Pascal nor the latest version of the report available from ETH themselves (which is basically a different scan of the same text i linked above): https://www.research-collection.ethz.ch/handle/20.500.11850/...
My guess is that they added these at some point later but didn't make a newer version of the standalone report (which is what i've seen in other places, e.g. http://pascal-central.com/standards.html refer as Wirth's last standard). I guess in practice it was simpler to have a single book act both as a tutorial and a reference, though i always though the 1973 report to be the last "standard" and didn't paid much attention to the other stuff released later.