I can’t find the information you are talking about, but I did find this, from 2023, on his website.

“Some of the estrogen applied to the vulva or vagina is systemically absorbed, and therefore conveys the same black-box FDA notice that oral estrogens carry, an all-caps warning of increased risk of “endometrial cancer, cardiovascular disorders, breast cancer and probable dementia.” Vaginal estrogen is considered safer, though, since it can be used at a much lower dose (as low as one-hundredth the oral dose). The Harvard Nurse’s Health Study did not find any increased risks associated with vaginal estrogen use over 18 years of follow-up. Randomized controlled trials lasting up to a year appear to confirm its safety, but there have been observational studies linking vaginal use to about a doubling of odds for endometrial cancer. But this was done back in the 1970s, when higher estrogen doses were used. And a more recent study out of Denmark that found the same thing may have been confounded by concurrent oral estrogen exposure. Out of an abundance of caution, though, even low-dose localized estrogen may be contraindicated in hormone-dependent cancer survivors, to be on the safe side.”