We have more than just soul related sorcery though, plenty of much more traditional magic that bring to question whether or not the user pulls from their own soul for those types of sorcery. We also have, in true Souls style, much more ambiguous wording for homing soulmass than you give it credit for:

"Life originates in the soul; no wonder the soulmass draws toward it"

Linguistically, there is a distinction between life, the soul, and the soulmass - with the soulmass being something that is drawn either towards life or towards the soul - but more likely towards life. The soulmass is considered a separate entity from the soul - as it is given a coined term as a name, it isn't soul mass as in a mass of souls, its soulmass as in the object known by that name.

If we consider that to be consistent from Japanese translations (of which I have no idea) then we need to look at reasons why Logan would be branded a heretic for his research in soul sorcery. We have plenty of soul sorceries that are not considered part of Logan's research, however the closer the sorcery gets to imitating life, the more heretical it seems to be considered. It seems more likely that Logan uses magic to imitate or recreate souls using whatever the mana/aether/juju is in the Souls universe in the context of him being branded a heretic for his work. If that is the case, then we have a more consistent rule that ties together sorceries of all types - where things like casting light and casting a "soul spear" are both done with universal magic juice that remains unnamed, however the soul sorceries draw their inspiration from the power and nature of the human soul.