Yes, there is a fallacy in “No person is illegal on stolen land.”
First is the category mistake and fallacy of equivocation: The statement conflates moral and historical claims (“for example, this statement usually means the land was stolen from Indigenous peoples”) with legal immigration status (the statutory definition of “illegal immigrant”).
Next is the fallacy of non sequitur. Even if the original acquisition of land was unjust, it does not logically follow that present-day immigration laws are nullified or that individuals cannot be classified as “illegal” under those laws. A person illegally within any jurisdiction is an illegal.
Of course, we can’t leave the discussion with mentioning the Fallacy of Appeal to Emotion. The phrasing is rhetorically powerful but substitutes indignation for argument, leveraging and alleging a historical injustice to dismiss present legal frameworks without actually addressing them.
The statement illustrates that the speaker is not aware of the difference between “Stolen Land” and the “Right of Conquest.”
Stolen Land is an illegitimate seizure — wrongful taking of property against established norms (theft, dispossession, or bad-faith treaties).
The Right of Conquest is an international law concept that allows territory to be acquired through military victory, followed by effective control and recognition by other states. While morally contested, conquest was treated as a lawful transfer of sovereignty at the time. This changed somewhat in the mid-20th century.
The original statement treats all conquest as “theft,” ignoring the legal distinction that conquest was historically normalized and recognized, whereas theft implies continuing illegitimacy.
This brings me to the generally accepted legal definition of ownership, which is the exclusive right to possess, use, and dispose of property, subject only to law. An interesting twist to this is that a possessor is entitled to possession against all except a person with a better title, which takes us too far afield from the question presented.