It appears to be true.
Ideally we'd like HMD Nokia to update the existing phones to support project Treble. The benefit of this is that Android updates could be made available for these phones in future.
However, this may mean that customers don't buy new phones. Since HMD need people to buy phones to make money it's possible they will not enable project Treble on existing phones.
Any new phone they release running Android 8 will have to support project Treble. So if you want a Nokia phone with project Treble then you will probably to wait until they release a new phone.