The indeclinable word yathā when it does not signify 'likeness' is invariably compounded with a word ending in a case-affix which is in construction with it and the compound is called avyayībhāva samāsa,
Thus यथावृद्धम् ब्राह्मणानामन्त्रयस्व invite every old Brahmana. So also यथाध्यापकम्.
Why do we say 'when not signifying likeness?'. For there is no composition when likeness is indicated. As यथादेवदत्तस्तथा यज्ञदत्तः As is Devadatta so is Yajñadatta.
This samása could have been included in the last sútra clause to, but making a separate sútra shows that the word यथार्थवचन there must be limited by the present sútra, to first three meanings of the word यथा given there and not to its fourth meaning. vis. there is no compounding when it means likeness.,