No, there is no contradiction because dBfs and dB are not the same thing.
A value of +6dBfs is not possible, because 0dBfs is the absolute maximum.
The specification -6dBfs therefore means that there is still 6dB of headroom until digital clipping.
The other recommendation +6dB refers to the SQ’s meters, which are similar to an analog console.
There is a value of 0dB (which corresponds to -18dBfs) and at +6dB you still have 12dB reserve for surprises before it clips.
This is a sensible value.
I don’t know where we could see dBfs on the SQ.
If the article was based on the SQ, it would have been better to write 12 to 18dB instead of 0 to -6dBfs.
See also page 113 in the manual.