The comment in the forum may be based on the Kitzur Shulchan Aruch (80:25) that prohibits molding and arranging a dish of onions and eggs:
איסור בונה שייך גם במאכל, כגון המגבן גבינה, או שמדבק פירות ומשוה אותם שיהיו יפין, ולכן כשחותכין בצלים עם בצים או עם חלב דג מלוח יש ליזהר שלא להשוותם וליפותם, אלא יניחם כמו שהם.
However, the comment does not represent a correct interpretation of this source, which is specifically referring to molding a mass of food to make it look nice. This halacha is based on the Magein Avraham (340:17), who writes that mushing pieces of food together into a decorative shape presents a problem of boneh.
Incidentally, some other types of decorative food preparation might pose problems of m'marei'ach d'rabbanan (see for e.g. Bi'ur Halacha at the end of 321, s.v. b'ma'achal) or koseiv d'rabanan (see for e.g. Sha'ar HaTziyun 500:20). However, simply arranging individual cookies in an attractive manner on a tray poses no problem on Shabbos or Yom Tov (so far as I know).