The situation is diachronically unpleasant. The -н- in обнимать and обнять is an innovation replacing older forms which would give обымать and объять. (Cf. 3rd person pronoun's forms with н after prepositions, which are themselves due to reanalysis of forms like вън его > въ него (> в него) and further analogy.)
For the verbs, -н-forms (понимать, донимать, etc.) have all but expelled from Russian -н-less-forms. However, объять (and, to a lesser extent, its imperfective pair обымать) was maintained by Church Slavonic (CS) influence, where -н-forms do not seem to have won (except for prefix в < въ < вън, cf. внять - внимать - внемлет). Объемлет is itself a CS correlate to обнимает with another imperfective affix (*-j > л, not a); aspectual distinction was yet fuzzy in CS, so объемлет was taken as, so to say, habitual present form (though not continuous, which would still be обымает expelled by обнимает).
Synchronically, however, Wiktionary's analysis seems to be the best: объемлет would have infinitive обымать but the verb has no prescriptively approved infinitive stem (and therefore no past as well). So the imperfective pair of объять is largely defective, having only finite present forms (and present participles объемлющий and объемлемый; I do not want to be engaged in a discussion of whether participles are verb forms). Of course, the situation does somewhat remind the situation with biaspectuals like казнить, женить and many -овать verbs, but the latter use the same form for perfective future and (necessarily imperfective) present.