"WordPress" didn't acquire Tumblr. Automattic did. Automattic (A8C) is the corporate entity behind WordPress.com and Tumblr.
The open source software can be found at WordPress.ORG and while the CEO of A8C does sit at the head of the board of the OSS project, his reach extends as far as others are willing to implement his ideas. Also to be fair, he does pay plenty of A8C employees to "contribute" to the OSS project on company time. This is largely how the Gutenberg project was pushed over the finish line.
The open source software can be found at WordPress.ORG and while the CEO of A8C does sit at the head of the board of the OSS project, his reach extends as far as others are willing to implement his ideas. Also to be fair, he does pay plenty of A8C employees to "contribute" to the OSS project on company time. This is largely how the Gutenberg project was pushed over the finish line.