Why circuit from antenna datasheet is like this?
The manufacturer's documentation was originally wrong.
The version you linked says it was from 2013.
They have fixed documentation from 2018 on their website here
The layout in the current documentation makes much more sense as the signal and ground are no longer shown as being connected:

One thing that isn't clear even in the new documentation is that you will need to jumper accross the gap in the feed line, potentially with a tuning element.
Antennas do not necessarily need to have one terminal floating. For example a loop antenna would have one terminal shorted. It's not really a red flag that this antenna uses a short instead of an open. Still the documentation doesn't make it very clear what is going on.
For a personal project I would definitely look for the best documented parts. Only use poorly documented parts like these if you really need to minimize cost in high volume.