Not true. The max that can be displayed is 32,767.
A terminal with 40,000 slots will show as 32,767. Adding 1,000 flights to that airport will then show 31,767. However, by using the "re-sync slots" button, the total will go back up to 32,767. This will continue until the true number of used slots surpasses 7,233, at which point the displayed number of slots will reflect the true value.
It's quite fascinating.
Interesting. I'm gonna create a new spamlines and used this to flood the market. Thanks.