Why does FPC compile this? (Btw. regardless of mode.)

program Project1;
uses Classes;
var
   O1, O2: TObject;
   B: Boolean;
begin
   B := O1 is TPersistent or O2 is TPersistent;
end.

Delphi throws a compile error and according to FPC docs https://www.freepascal.org/docs-html/ref/refch12.html it should not compile in FPC either.

Ondrej