What they mean is that a search three times three times gives lexicographically ordered output of strings in trie.
But yes, you’re right, it assumes that all siblings at this level are visited in lexicographic order, and this is far from given, especially with large alphabets, where it makes sense to implement a node child table through a hash table.
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, trie , arent , - trie node , trie - O (n) - -, , O (n log n).