It seems that it is more about holding them than declaring them and apparently they were "on public record throughout his time as an MP from 2019 up to this year", so failure to declare doesn't appear to really come into it. Apparently he has been temporarily suspended. Interestingly, he could still have been considered for PM, but was not renominated. (Personally, I hope he is at least allowed to return as an MP.)
Not much recent news on this. ????♂️
https://www.aljazeera.com/news/2023/7/19/thai-court-suspends-pita-as-mp-as-parliament-votes-on-new-premier
https://www.thaiexaminer.com/thai-news-foreigners/2023/07/19/pita-limjaroenrat-suspended-mp-constituonal-court-and-blocked-pm-parliament/