Leaving aside X' theory¹, an easy thing to do, since it makes far too many invalid assumptions and doesn't work very well to start with, the question of whether English modals are tensed or not depends on what one means by "tense".
Traditionally, tense is a verbal inflectional category, like the Latin, Greek, or Sanskrit tenses. English has only two such, past and present, aside from defective verbs like beware and born. English modal auxiliary verbs, as noted, are not inflected for tense or anything else, except for the special cases that Greg Lee mentioned in his answer, where the old preterite/present modal stems can still occur in that sense. But these are very rare and don't count as inflection, really, since it's simply an opposition.
Many theories, however, require a tense marker on every verb (one of the invalid assumptions mentioned above), and so they put one there. Of course the marker is invisible (as well as inaudible), just a local variable that's needed to process the input and eventually disappears. Why go to all the trouble?
¹Pronounced "X-Bar Theory" by the cognoscenti, which gives you some idea about its degree of abstractness and contact with linguistic reality.