No, Niantic is not developing Harry Potter: Wizards Unite. Wizards Unite is developed by Portkey Games, a newly established Warner Bros. development label created in 2017. Niantic's role in development of HP: Wizards Unite is to provide map data, Points of Interest data sets and AR expertise.

Warner Bros officials have clarified this relationship on November 9, 2017, in a series of articles published on Pottermore. Relevant links: #1, #2.

Portkey Games is an umbrella studio created for the purpose of publishing Wizards Unite and future development and publishing of mobile and console based Harry Potter games. The company was registered in 2017 as an LLC in Delaware.

The following screenshot shows legal information about Portkey Games:

Portkey Games

The official Portkey Games website is:

The official trademark information/statement is:

Portkey Games © 2017 Warner Bros. Entertainment Inc. All rights reserved. Harry Potter and Fantastic Beasts Publishing Rights © J.K. Rowling. J.K. ROWLING’S WIZARDING WORLD is a trademark of J.K. Rowling and Warner Bros. Entertainment Inc. Portkey Games, Harry Potter and Fantastic Beasts characters, names and related indicia are trademarks of and © Warner Bros. Ent. All rights reserved.