If the library has two vocabulary learners, only one of which does the described thing, then isn't it unambiguous which implementation within the library the question refers to? And wouldn't it be ambiguous to instead say "how does Unigram do it" without referring to any particular implementation?

Anyway, the paper says "Frequent substrings can be enumerated in O(T) time and O(20T) space with the Enhanced Suffix Array algorithm (Nong et al., 2009)", which is hilariously underspecified, at least in part because a suffix array algorithm isn't a top-k algorithm.