Perhaps they mean than SOME specific instruments (e.g. 1 or 2 clarinets) from the wind family and ALL (usually 2 to 4) trombones are absent.

In the early symphonies, the default orchestra of the period, i.e., oboes, horn, and strings, predominates. Occasionally one find trumpets and drums, and even specialty instruments like the piffaro (a double-reed member of the oboe family) and English horn (No. 18).
http://www.arkivmusic.com/classical/album.jsp?album_id=16056

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As mentioned before the ensemble of the score is fully symphonic with the London Voices choir complementing a large symphony orchesta and few selected specialty instruments adding their unique timbres into the mix
http://www.jwfan.com/forums/index.php?/topic/21665-score-har...
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Good classical music has costs that the public often doesn’t know about — high costs for copyrighted parts and scores (below) for recent works, and specialty instruments such as viols or oboes d’amore and portative organs for early music.
https://welltempered.wordpress.com/tag/vaughan-williams/?ei=...
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Alto and bass flutes also exist, but are specialty instruments.
http://www.musicforthelistener.org/unit1/chapter2.php
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