I think you may be referring to something else. AFAIK
@Harry Callahan is talking about flare, in the sense of 'veiling glare', which does indeed reduce overall contrast by illuminating shadows that would otherwise remain darker. See e.g. this explanation by Imatest:
https://www.imatest.com/docs/veilingglare/
What veiling glare
will do is create density (on a negative image forming medium) in shadows. As such, it will also interact with the toe behavior of the medium as there's a point where this additional shadow exposure sensitizes the medium in the same way a preflash does, so it will allow the capture of
some shadow detail that would not be recorded when using an optical stack with less glare. Note that this only happens in a select part of the toe region of the film.
What veiling glare will
not do is create detail/contrast in those shadows, or significantly affect straight-line or shoulder behavior of the medium, since there is no significant decrease in exposure happening in those parts of the curve.
All combined, an optical system with a large amount of veiling glare will reduce overall contrast by making shadows lighter, possibly capturing marginal detail in those areas (albeit still with low contrast/poor separation). The severity of the effect changes with the amount of glare. As such, the effect is relatively easy to reconstruct by a simple A/B test with a glare-prone lens with and without hood and a light source just outside the image frame, and then comparing the captures.
Whether it is an effective strategy to use a lens that is particularly prone to glare as a means to compensate for undesired film curve behavior (or vice versa) is up to the individual photographer or cinematographer to decide. Personally, I think it's a tricky strategy since it is very prone to per-shot conditions, specific glare behavior of the lens and the specific characteristics of the film that need to be addressed. That makes it a rather difficult to predict set of interactions. I'm sure high-end cinematographers could expound lengthily on how to make thos controllable, as it's a phenomenon that can indeed be exploited deliberately for creative purposes.
For the still photographer and especially amateurs, I can imagine that it can simply be a fun and somewhat effective 'workaround' to combine a flare-prone lens with a film that has a pronounced S-curve behavior and get pleasing (albeit somewhat unpredictable) results. Personally, I'd rather choose to use a properly coated lens with decent optical characteristics and avoid the toe of the curve in those cases where it's needed by simply adding some exposure, perhaps combined with some reduced development.