I have been doing iengar yoga, the one which uses blocks, straps, foam rollers (noodles), chairs and bolsters.
Yoga helps with overall flexibility but it is not the only way to improve flexibility. Improving it surely benefits the ground game.
The thing that stands out for me regarding yoga effecting my grappling is that I am better at managing my energy since starting yoga. Specifically by being more aware of my body, I actually relax un needed muscles until they are needed as opposed to being in a constant state of tension all while rolling. This way, after relaxing some parts I can be more explosive with a burst of energy that would not have the same explosiveness without the previous rest.
On the flip side, it has balanced or reversed some of the tension and alignment issues that BJJ causes.
For example tight quads, pelvic tilt and bowed legs.
It seems when we are in certain positions a lot, we tend to hold those positions to some extent after we are finished using them.
A good yoga instructor will notice these imbalances and correct them with either an opposing movement or position or a stretching or a stimulating one.
IME the yoga corrects the postural and symmetrical imbalances that combat arts create.
It has helped me further rehab some injuries after PT was completed. It has really helped reduce and almost eliminate knee pain from hamstring tendon strain and meniscus tear. Before yoga I would walk 3 blocks and be in pain, after 6 weeks of yoga I could walk indefinitely without knee pain. To a smaller extent it has helped my ROM in other areas.
These are my experiences, your mileage may vary.