If you take proviron when natty, it eventually suppress your endogenous testosterone production (yes i know the study with 150 mg per day and no suppression...). If you suppress testosterone, you remove the substrate for estradiol production. That, coupled with the weak AI effect of proviron, explains where the theory of antiestrogenic property of proviron comes from.
The reality is: proviron binds avidly to SHBG, freeing up everything is bound to these plasma proteins, which are: DHT, testosterone AND estradiol (the latter has the lowest affinity).
Now we have two scenarios: first, people with higher 5-alpha reductase activity, who are going to have a better androgen/estrogen ratio, so a net (mild) antiestrogenic effect; second, people with higher aromatase activity, who are going to convert excess of free test into estradiol.
Last but not least, mesterolone (proviron) has its own activity as a neurosteroids. One of its metabolites, 1-methyl-androstanediol, has GABAergic property and it can modulate estradiol receptor activity in the brain acting like a SERM.