incomparable, merely
啻 centers on the idea of being beyond comparison: something so exceptional it is incomparable, or conversely, something so ordinary it is merely what it is. The two senses are linked by the notion of a single, unmatched quality.
啻 combines 帝 (emperor, supreme) with 口 (mouth), likely suggesting a supreme or singular utterance. The exact historical development is uncertain, but the components point to an idea of unmatched quality.
The emperor 帝 speaks from his mouth 口, and his word is incomparable—no one else's words can match it. That single, supreme statement is merely his, yet it stands alone.
For シ, imagine the emperor's incomparable word is a simple 'she'—just one syllable, merely 'she', yet it carries unmatched authority: she -> シ.