Is this usual?

Very typical.  I use it sometimes, and I see it all the time with other organists.  I'd be careful with it as it isn't readily ergonomic--especially for smaller hands, but there are plenty of places in the literature where it makes sense and can be safe enough to depend on in a concert setting, but it's highly dependent on the organ as well, since different organs may have different distances between the manuals, different key widths, etc.

It's also common to see thumb on one manual and pinky on the one above.  There are plenty of fingering combinations that are impossible, but there are a few that feel somewhat natural.

In college, my first musicology course was taught by a dear, late friend who was a professor of organ (and harpsichord).  Once, he decided to quickly play some baroque music at the piano to demonstrate something about counterpoint, and another friend and I--who both studied piano--couldn't help laughing as this extremely brilliant, knowledgeable professor spent several minutes trying to sit at piano as though it was an organ.  He had no idea far back to the put bench, no idea where to go with his legs, and no idea where he should have his center of gravity.  So, yes, you're absolutely right, those three are totally different in how you play or sit at them.

Also, yes, I've noticed that organists--particularly those that work with tracker organs or are harpsichordists--have very heavy touch.  Harpsichords require a lot of wrist strength and confidence, and you strike the key in generally the same place.  Organs--especially if they're tracker organs (as opposed to electrical or pneumatic actions)--have the wind pressure of the instrument working against you, so you're pressing a key against how ever many inches of wind pressure is about enter the pipe once its open (large classical French organs changed this because there was so much wind pressure for full organ, air pressure was added to the console and manuals to help the organist push back against it).  Furthermore, neither instrument has any benefits from pressing a key in a different place or a different way, since in most cases, the organ or harpsichord will sound (essentially) the same regardless of technique.  Whereas, a piano has a very different sound and expression based on the angle of the finger tip and the place on the key surface that is pressed.  So, yes, both organ and harpsichord will lead to a much heavier technique, and like my professor, organists almost always sit at a piano very vertically because they need that balance to keep their feet free for pedals and get their strength from their wrists, while a pianist (usually) leans into the keys and gets their strength from their shoulders.

A brilliant example of harpsichord technique from Karl Richter, who I feel is probably the best organist of modern times: https://www.youtube.com/watch?v=ERISHVNCmAk

Note how high his palm is above his finger tips, which would be almost a hopeless technique at the piano, but it's perfect for contrapuntal harpsichord music.