No, Los Angeles County's new tabulator isn't open source. Look what happened when someone tried to request the source code for their "open source" system (as LA County's press release called it). LA County replied that it's "exempt from disclosure" for a whole host of reasons (2 pages worth):

https://osvtac.github.io/files/meetings/2018/2018-09-13/pack...

https://osvtac.github.io/meetings/2018/2018-09-13/agenda

In contrast, the City and County of San Francisco is working on developing and certifying an open source paper-ballot system.