The Head Judge may choose to not issue this penalty if they believe that what the player wrote on their decklist is obvious and unambiguous, even if it is not the full, accurate name of the card. This should be determined solely by what is written on the decklist, and not based on intent or the actual contents of the deck; needing to check the deck for confirmation is a sign that the entry is not obvious.
So the issue is fundamentally what a player could conceivably mean by “23.” As Ernst Jan Plugge correctly points out, format is potentially important here due to the existence of Snow-Covered basics, but when we wrote this scenario, the intent was for it to be a Standard event. With that context, there are only 5 cards of which a player could play 23 copies, and those are the basic lands. Of the basic lands, only “Island” makes sense in the context of the rest of the list, which is mono blue.

Some consideration has been given to the possibility that “23” may mean “23 assorted lands.” While this could theoretically happen, it is not consistent with the way players write actually deck lists. If you see a list that says “4 Bolt” that means “4 Lightning Bolt,” not “3 Lightning Bolt, 1 Forked Bolt.” In much the same way, it would be very easy for a rushed player to write 23 and leave it blank with 23 copies of the same card. It would not make nearly as much sense for him to write “23” when he meant “17 Island, 4 Temple of Mystery, 2 Radiant Fountain.”

As such, it is the opinion of the Knowledge Pool team that 23 unambiguously means “23 Islands” and the deck list should be corrected without penalty. No follow-up deck check is required.