In a paper on chemical periodicities of elements, Cao et al. show the number of valence electrons in a periodic table (https://doi.org/10.1515/pac-2019-0901). Apart from Boron and Aluminum shown as group 3 and 13, the table matches what is taught about main group elements in textbooks except for the entry for fluorine.

Fluorine is listed as 5 "active" valence electrons, implying perhaps that the 2s electrons do not participate in bonding. Why is fluorine treated differently than oxygen (or does oxygen make compounds where the 2s electrons are more involved in bonding than those of fluorine)?

If I look at the effective nuclear charges experienced by the 2s and 2p electrons in nitrogen, oxygen and fluorine, there is no jump in the energy difference that would place fluorine in a special category (values found on Wikipedia from Clementi et al 1963 and 1967):