The author cited, Michael Franz, was one of Wirth's PhD students, so what he relates is an oral communication from Wirth that may very well never have been put in writing. It does seem entirely consistent with his overall philosophy.

Wirth also had no compunction about changing the syntax of his languages if it made the compiler simpler. Modula-2 originally allowed undeclared forward references within the same file. When his implementation moved from the original multi pass compilers (e.g. Logitech's compiler had 5 passes: http://www.edm2.com/index.php/Logitech_Modula-2) to a single pass compiler http://sysecol2.ethz.ch/RAMSES/MacMETH.html he simply started requiring that forward references had to be declared (as they used to be in Pascal).

I suspect that Wirth not being particularly considerate of the installed base of his languages, and not very cooperative about participating in standardization efforts (possibly due to burn out from his participation in the Algol 68 process) accounts for the ultimately limited commercial success of Modula-2 & Oberon, and possibly for the decline of Pascal.