The intent needs to be clear in the licence text itself. In a dispute, courts will look to intent somewhat, but generally the law assumes that when parties to a contract or licence or deed state their intentions in the form of written text, that text will be primary in understanding the content of the agreement that was made.
The authors of legal texts don't have magic powers. Let's say you buy a product from a company, C. They wrote a contract which you both agreed to. They are a big company, and you are just an individual person, so your bargaining power is pretty minimal compared to them, and basically amounts to "take it or leave it". So you buy a thing on their terms. Some term of the contract later turns out to be unclear, so you ask them to clarify it. Is the clarification they provide going to be beneficial to their interests or yours? Theirs, obviously.
Stallman's interpretation of the GPL in the case of uncertainty or lack of clarity is going to be to read it in such a way as serves his goals: making sure there is more free software in the world, and protecting the Four Freedoms. A corporate lawyer working for Oracle is going to interpret unclear provisions in a perhaps rather more conservative way, especially if it financially benefits Oracle. The fact Stallman wrote the license doesn't give him any particular special hermeneutical status when working out how a court will interpret it.
"Hey, just trust us" isn't useful either, because there's a difference between being asked to trust randomhacker849 and being asked to trust a multinational megacorporation like Google or Oracle. While randomhacker849 may follow the broad spirit of any particular open source licence and expect licensees to follow the broad spirit too, I'm pretty sure Oracle are going to follow the exact letter if doing so is in their financial interest.
The whole point of legal instruments is the need for trust is reduced because you have rights and obligations that a court will enforce. And the whole reason we turn agreements into written legal texts is precisely because "hey, just trust us" doesn't scale—when millions or billions are on the line, you want those agreements formalised.
(I included Apache and Mozilla because they represent examples of open source non-profit foundations, not because they use the GPL, incidentally. Other than GNOME and FSF, most of the projects that have their own foundation—LibreOffice's Document Foundation, Perl, Python, Mozilla—have permissive licenses rather than copyleft. The point isn't about the wording of the text, but more the question of "if I were going to rely on a disputed bit of license text in my use of this software, what risk would I face of litigation?"... and that risk differs considerably between randomhacker849, a non-profit foundation, Stallman/FSF, and a huge scary litigious corporate beast like Oracle.)