He's eligible because this will be the *fifth Rule 5 draft since his official contract signing back in July of 2018. 2018 counts as a "qualified season" for Rule 5 purposes; because he signed before the DSL season ended, it counts regardless if he wasn't assigned to play in league games until the next year. Had he signed after the minor league regular seasons ended in 2018 then that year would not have counted as a "qualified season" even if he signed before that first rule 5 draft (say October instead of July for example).
Rule 5 status has nothing to do with Rule 9 (minor league FA) team control, which allows the Reds too control him through at least the 2025 season if they wish to do so (assuming for hypothetical purposes that he isn't protected and he's not taken by another club via a Rule 5 draft). The canceled 2020 minor league season does count for Rule 9 purposes.
*The lockout canceled rule 5 draft counts as a year for draft purposes as well.