Not true, chemosynthesis can occur deep underground just about anywhere were there is a latent heat source or chemical energy. It helps that its close to the vent only because of the motion of the water and thus the avialabikity of nutrients increase.. The rate of chemical reactions in biology 2 fold per 10'c, sobthat the more diificult the thermodynamic barrier to over come the more likely heat and catalysis will overcome the barrier. But also as heat increases so do unwanted side reactions, which is a particular problem for complex life forms. Most of these live in the thermoclines surrounding the vents. Thermally created chemisynthetic life will probably never push the development of really complex life, but it could create commensal communities of single cell organisms.
Local black bodyradiation is a problem, if the object is hot enough to create hv required to create complex high energy bonds (which is not hard to do) but keep them stable long enough to fulfill function you are talkiang about temperatures in the 1500 to 5000'c range, which at production densites stable water cannot exist. Light brings those high wavelengths into contact with water but at low enough densities not to boil the water off at once. We can think of light as adding powerful talents to biology, augmenting the default talents that come about after coalescence into planet, but that for these augmentations to take place there is a need for residual water and soluble micronutrients.