According to Flyers history, Clarke always wore #16 except for that one night. It's perhaps some training camp thing.
I suppose the list of worn numbers might only encompass the regular season... If a player wore a number in Training Camp or Rookie Camp it might not be listed. The FlyersHistry site has JvR as only wearing #21, and I did see him wear #10 in his first Rookie Camp. If Clarke had #10 in Camp and was assigned #16 on Opening Day I assume only #16 would be listed... But I don't know if we can know that for sure. If anyone runs into Clarkie at a bar, maybe we can ask him as they buy him drinks.
Rookie Camp is not training camp. I don't even know if pre-season counts when they list numbers. Oskars Bartulis isn't listed under #38, but he wore it in pre-season and in practice too I believe until they changed it the day he was being put into a game.