There is a good reason for questioning the "core document" as it is the more pertinent one to the general order and aim of the EU. The construct is not very convincing and is very dangerous, though, it was the Maastricht Treaty that has so far caused the greatest harm to the populations. It is also quite clearly rather doubtful in a legal sense. It is almost identical to the Constitution of the EU that the was voted against in referendum. After that the Euphiles were afraid that putting a slightly modified text to the People for a second chance like they have done before, When populations refused. So they gave the document a new name and sighned it as a Treaty in stead of calling it a Constitution. Very sleazy, indeed and probably illegal from a more objective level. Be that as it may, the legitimacy of the EU was undermined.
But the hypothesis that the Lisbon Charter is not a constitution made it easier for the German Bundersverfassungsgericht (Higher Court) zu accept it as constitutional, as it could say that it was not a "Constitution" and therefore German law and the Bundesverfassungsgericht remained sovereign, without which they would have had to judge it unconstitutional.*) The European courts and the Bundesverfassungsgericht have been very careful not to accept a case on which they would have to confront each other frontally. Instead they are running a complex dance in which the population is being readied for such a seeding of sovereign in future, when it is deemed the right time.
In fact, of course, the Lisbon Treaty is being treated as a constitution by the politicians and bureaucrats. The opinions of the populations and hypothesis of the high court be damned.
*)They floated the idea that "asking the voters" as is stipulated for the transfer of sovereignety to an other constitution might not require asking the people, as Germany was an indirect democracy and so the Parliament might be able to legitimize a new constitution. This went down in smoke.