As someone who consumes Huel Black for 2 of my 3 daily meals and have been consuming Huel for over a year now, I think I can speak on the topic.
I’ve dropped 93 lbs over the past 13 months and have been Vegan since 9/2019. While I haven’t had any tests done, I would say that my testosterone levels have increased significantly, as has my strength and sex drive.
In addition to the two daily Huel servings, I snack with Huel Bars (no clue if they have flax) and have a bed of greens with ground flax mixed in with every dinner meal at least 5 days a week for over a year now.
No idea if any of the above is of value to you, just speaking from my personal experience. I think any study that speaks to the contrary is BS in my opinion.