Disclaimer: IANAE (I am not an expert)
The impact will also vary greatly depending on your current fitness/level of training. If you are completely untrained you would make progress training at any % of your FTP (other than 0!). If you are highly trained / near your peak potential fitness, you may not make any improvements or may lose fitness. If you have peaked/tapered you will lose fitness even training at 100% of your FTP.
I’m not sure that training at 100% of your FTP is optimal for every individual (depending how their body responds to training stress) - but I’m completely speculating here.
TLDR: Unless you’re highly trained you will gain fitness at 5% of your FTP, but possibly slower (and maybe even faster).