Well we'll never know for sure but not the case in the UK
You can't really tell from that article. This is all it says:
According to Chart-Track Gran Turismo 6 sold only around a fifth of the first week total for 2010’s Gran Turismo 5, which holds the record as the fastest selling game in the series.
No volumes are mentioned, only that it was 1/5th of the record-holder GT5. According to their methodology their statistics are retailer based, so do they count digital copies sold on the PS Store?