Bhashya
मासानां प्राप्तिं श्रुत्वा तदवयविनः संवत्सरस्यापि पूर्ववत्प्रा-प्तिरापद्येते ।अतः तत्प्राप्तिः प्रतिषिध्यते— नैते संवत्सरम-भिप्राप्नुवन्तीति ॥
In plain words
Once the reaching of the months is heard, it would naturally follow that the year, of which the months are parts, is reached in the same way. Therefore, that very attainment is denied by the text: 'They do not reach the year.'
Close to the Sanskrit
Upon hearing the attainment of the months, the attainment of the year as well—its whole—would result just like before. Hence that attainment is prohibited: 'They do not reach the year.'