'Sirat-e-Siddiqui' is clarified by the author himself i.e., 'Fana fi Rasool(pbuh)' losing oneself in footsteps of prophet Mohammad(pbuh). Why do you need to dwell on any publisher's footnotes.
So 'siddiqui' doesn't necessarily mean pertaining to 'Hazrat Abu bakar siddiq RA'.
I need not talk about footnotes of the publisher but the footnote is self explanatory and leaves no room for any controvery i.e.,
'Complete devotion to the Holy Prophet such as shown by Hazrat Abu bakar siddiq'
It doesn't mean that Abu bakar siddiq claimed prophethood or everybody following the footsteps of the holy prophet will certainly get prophethood.
Quran says 4:69 "And whoso obeys Allah and this Messenger of His shall be among those on whom Allah has bestowed His blessings, namely, the Prophets, the 'Siddiqeen', the 'Shuhada', and the Righteous. And excellent companions are these"
So as we can see there are four blessings of Allah for the one who follows Allah and His messenger(pbuh) and prophethood is one of them. 'Siddiqiat' is 2nd next to prophethood in spirtual status and can only be attained through submitting to Allah and following the footsteps of the Holy Prophet(pbuh).