The data behind all of this is for women taking progestins as part of their HRT. Progestins are chemicals that are absorbed well, which can also be $patented$. They do provide some breast tissue protection, but do not have the cardio protection of progesterone. All steroid hormones are transported to the cell nuclei where they alter gene expression. There are multiple gene expression effects for any one hormone.
It is obvious that progestins do have all the gene expression effects of progesterone. Progesterone has very low oral bio-availability and as a trans dermal, it can be ineffective in terms of proper balancing with estrogens and it can take some effort to find out what works best for any one woman. Transvaginal works well, and is very protective of endrometrial overgrowth, fibroids and downstream cancers. [do not included cervical cancer in this, totally different concerns]
A woman might do well with HRT based on progestins, which are very inexpensive, and also applying 2% progesterone cream. I have not yet seen that described. The bio-identical purists would not do this. However, costs are often a issue. A women can get a years supply of estrogen and progestins as tablets for $80.
Note that breast cancer happens all of the time in post menopausal women. Many of those cases will involve cells that started a cancerous cascade of changes decades before. Women start to loose progesterone levels in their 30's and become estrogen dominant. This results in the changes to the uterus cited above, as well as painful and prolonged periods and cramping, PMS and other mood problems. All women should be using progesterone cream in their 40's to prevent or resolve such problems and to protect their breasts.
The basic issue is balancing the effects of estrogens with progesterone. Most docs would not understand the complexities of this, even if they were using the right hormones.
The issue has become complicated as there is a lot of deception in the 'industry'. When the issue came up around 10 years ago, the drug companies deflected blame by wrongfully pointing their collective fingers at HRT, when it was really an issue of industry designed chemicals that were primarily created to obtain patent rights. The issue was cardio, now it is breast problems.
There is no large study data one way or another for HRT based on E+progesterone. Estrogen only HRT seems to be suicidal.
Note that oral birth control [OBC] has the same issues. OBC represses a woman's HPOA and thus progesterone becomes displaced by progestins. Thus we could expect to see increased cancers from decades of OBC use. Add that to natural progesterone decline and you have a major problem.