Complex sentence: These schools accept that the Vedas were "seen" by the Rishis, however, that the Rishis merely "saw" the Vedic hymns does not necessarily mean that the hymns were composed by them.
Split into simpler sentences: These schools accept that the Vedas were "seen" by the Rishis. However, that the Rishis "saw" the Vedic hymns does not necessarily mean that the hymns were composed by them.