How do historians respond to the accusation, that historical explanations which make use of teleological assumptions (i.e. john did this thing because he thought, believed, desired, feared some thing) are strictly false, given what we now understand about human action via various cognitive sciences?
This opinion is expressed by philosophers like Alex Rosenberg. He states his view in this podcast: https://youtu.be/IBIk-S3g4T4.
The example you give is not a teleology. Historians often make arguments based on what historical actors may have thought, believed, etc. Those are not teleological. The strength of such arguments (like any historical claims) lies in the evidence presented. With the right sources we can certainly know what historical actors believed, thought, feared, etc. Building an argument about the past based on such claims is perfectly valid.
Teleology takes the historical outcome as a given and tends to interpret evidence in that light. Because of this such interpretations overlook historical contingency, for example if you took the outbreak of WWI as a given you would ask different questions and interpret sources differently than if you sought to understand why the war broke out and attempted to understand both the forces propelling European powers to war as well as those forces that mitigated the outbreak of war.
History happened the way that it did, but historical events do not have inevitable outcomes. The historian needs to examine context and contingency to understand why events unfolded the way that they did.