That's not the point. The point is that the community will be able to do it and auditors' lives will be much easier, which benefits everyone because it vastly increases the likelihood of an issue being found.
> The idea that ROMs from questionable sources make your device safer sounds very strange to me.
On Android by default most vendors ship a lot of bloatware and have demonstrated almost infinite incompetence or malice of both.
While the developers "custom ROMs from questionable sources" (XDA forum threads) may not be experts in their fields at all and are quite likely to misconfigure the software possible creating some new holes, at least those images are compiled from open sources and are not the terrible manufacturer OS (I wouldn't onlike bank on stock Xiaomi software).