Especially the lack of separate compilation units was a weak point of the original Pascal, as also noted by Kernighan. This was fixed later in UCSD Pascal (1978) and of course by Wirth himself with Modula-2 (also 1978).

But that said, it's strange that Kernighan wrote the above article ("Why Pascal Is Not...") in 1981, i.e. three years after the introduction of UCSD Pascal and Modula-2.

Does anybody here know the background of that article? Why was Kernighan feeling the need to defend C against a 13-year old language which was already not in use anymore in its original form at the time the article was written? Was C under attack and risking to lose its popularity?

It should be noted that even when Kernighan wrote that, with the exception of the stylistic bits (e.g. where you put the semicolon), most of the issues he has didn't exist in the Pascal dialects that people used. His comments were about standard Pascal but few really confined themselves to standard Pascal - even the standard itself didn't do that, IIRC it explicitly mentions that implementations are going to extend it as that was an expectation at the time. In general Wirth's languages weren't meant to be taken as a gospel but as a base to expand from (even his own Oberon system slightly extends the language described by his Oberon language report).