Yes, you do need FS9.1 installed, there's no way around this, because the scenery needs some things to be at specific locations in memory, which are different from 9.0 and 9.1 so, under 9.0 it would just crash, if we allowed an installation without the upgrade installed.
Also, 9.0 is less stable than 9.1, especially when dealing with terrain, that leads to CTD due to memory leaks more easily than 9.1 but, since users that buy payware, always complain with the 3rd party developer for a CTD, not wanting to taking into account the sim itself might be bugged, we don't support 9.0 in order not to have to deal with such issues.
It's wrong insisting to use 9.0 because, there are no reasons whatsoever not to update to 9.1, mainly because of stability. Installing 9.1 DOES NOT require to reinstall any addon, and it's usually painless, of course if you are updating it over a 9.0 ORIGINAL FS9.EXE file (meaning, no nocd patch and no FS9.EXE patched >2GB). You can always patch your 9.1 FS9.EXE AFTER you install the update, if you need to.
And, it's not true that no other addons requires 9.1 installed. The PMDG MD-11, for example, requires it, Traffic 2005 from Justflight requires it, but there might be others as well.
And note that, we don't check the FS9.EXE version to see if you have 9.1 installed, we check G3D.DLL only.
Which means, if you have specific reasons for not wanting to install the update, with the most prominent one being having access to a 9.0 nocd crack but not the 9.1 one, you can still apply the update and, if you really want to use a 9.0 crack (which is not a good idea anyway, because mixing 2 versions might be even worse for stability that not applying the update at all), it will work with our installers, because they don't care which FS9.EXE you have.
They used to check the FS9.EXE, but the current version checks only G3D.DLL, because we found many users messed up their FS9 install by mixing the whole set of .DLL from 9.1, with the cracked FS9.EXE from 9.0 so, we no longer check the .EXE
