Neither of those libraries uses any extensions. TweetNaCl is a famously minimal implementation of NaCl in portable C by the NaCl authors. Monocypher is a library 'loup-vaillant wrote as a sort of side project. TweetNaCl does see some use, but libsodium is the most popular NaCl implementation, and libsodium's author provides his own low-footprint misuse-resistant crypto API separately.

Neither TweetNaCl nor 'loup-vaillant's library really addresses the low-footprint concern.