in need 的意思???

This is a very good question.

I think your suspicion is correct. "in need" does have a specific meaning for people in poverty."in need of" would have been more appropriate in this situation.

However, for the posting, I suppose most people can get the meaning of "in need" as "in need of the seat", althogh grammatically it could have been stated as:
"please offer your seat to anyone in need of it", or

"please offer your seat to anyone who needs it".

意思係"在需要" 　．．．．．．．．．

Please offer your seat to anyone in need=

Please offer your seat to anyone, whoever needs a seat - i.e eldery people, children, people carrying babies or young children, pregnant women or handicaps,

It also has another meaning - people who suffer poverty and hungry

Please offer your seat to anyone in need.=请为任何人提供您的位子在需要。

咁in need可能係「有需要 」