But I wonder: since, as you have shown, import is special-cased so that invoking it when it’s not defined is passed over in silence, why is it necessary or useful to add it to the symbol table at all in this case?

That's probably just an implementation detail.

It is common for symbol table entries to be created whenever the name is referenced. For instance, the following code...

sub foo { bar(); return $baz }
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adds entries for foo, bar and baz.

$obj->non_existent_method;
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dies but after creating an entry for non_existent_method.