Well, obviously they both have a phytoestrogens, however they both contain different phytoestrogens from one another. Soya has isoflavones, Genistein and Daidzein respectively, flax has various lignans instead.
There are countless foods that already contain lignans, such as many vegetables and grains, such as oats!
They are there naturally anyway in your diet, no matter if you eat Flaxseeds or not. I do see your point regarding the fact that flax does have a denser amount of lignans, but as a whole flax is dense anyway, from a calorie and nutritional standpoint, it’s not exactly like you’re eating an entire kg of flax.
There is only 12g if flax per 100g of huel apparently, so if you ate 2000 calories worth of huel that’s only 60g. Again it’s about perspective, yes flax has 3 tines the amount than soya, but it’s very likely if you’d eat a soya product that you’d be eating more than 3 times the amount that you would be able to in flax seeds.
Also worth pointing out that Chia seeds and Sesame seeds both contain lignans, no one talks about those for example.
Hope this helps! 