Shortfills arent, they were the industry's workaround to the 10ml bottle size limit. One sees tpd compliant 10ml eliquid reduced all the time, so not sure whether something has changed.
To be honest there is a chance that Brexit may have changed this since it's a TPD stipulation that I can't find has been explicitly included in our TRPR regulations, but the last time we included TPD registered products in a discount code on here, someone reported us (and another retailer) to the ASA and we got a slap on the wrist for it and promised not to include TPD products in promotions moving forward. We haven't done so since, but I will look into in case things have changed.
The TPD only applies to 'nicotine-containing products' - anything that doesn't contain nicotine, be it a shortfill, a pencil, or a toaster or whatever else, is exempt from the regulation and therefore cannot comply with it by definition.
That said, it is possible, as is the case with the Mouth To Lung 30ml shortfills on our site, that a product can be exempt and yet still comply with the regulatory standards. Despite their exempt status, the MTL range has been lab tested to TRPR/TPD standard and could legally be sold as a fully compliant 10ml, however we found the 30ml Shortfill + NicPack to be more popular due to the larger bottle size & strength flexibility.