Your reading is correct.
When casting Summon Lesser Demon, you have to roll a d6: if you obtain a 1 or 2, the table says that the spell summons exactly 2 demons of CR 1 or lower. Since manes' CR is 1/8, and 1/8 is less than 1, the DM may make the spell summon 2 manes, per description:
The DM chooses the demons, such as manes or dretches, [...]
One may find more information in the Sage Advice Compendium about DM choosing the creatures summoned by a spell once the CR has been established:
When you cast a spell like conjure woodland beings,
does the spellcaster or the DM choose the creatures
that are conjured? [...] Some spells of this sort specify that the spellcaster chooses the creature conjured. For example, find familiar
gives the caster a list of animals to choose from.
The design intent for options like these is that the spellcaster chooses one of them, and then the DM decides what creatures appear that fit the chosen option. For example, if you pick the second option, the DM chooses the two elementals that have a challenge rating of 1 or lower. A spellcaster can certainly express a preference for what creatures shows up, but it’s up to the DM to determine if they do. The DM will often choose creatures that are appropriate for the campaign and that will be fun to introduce in a scene.