That people accept mirrors gifted for free is not strong evidence of their superior quality, when you get something for free you can make do with something that's not really what you want.
As an example, the MMT observatory was so happy with the "cobble six telescopes together" design foisted upon them by the donation of the mirrors that within 8 years of completion they were officially[1] reporting they hoped to replace them with a single mirror by 1993. Honestly I think the MMT is the only telescope I've heard of where the main mirror was so bad they opted to replace it with a completely different design within 20 years.
Anyway, the Hubble mirror has a different focal length than the KH-11 so can't have been used as is, it would have to be reground and polished for the new curvature and so it would have been essentially just a mirror blank.
As an example, the MMT observatory was so happy with the "cobble six telescopes together" design foisted upon them by the donation of the mirrors that within 8 years of completion they were officially[1] reporting they hoped to replace them with a single mirror by 1993. Honestly I think the MMT is the only telescope I've heard of where the main mirror was so bad they opted to replace it with a completely different design within 20 years.
Anyway, the Hubble mirror has a different focal length than the KH-11 so can't have been used as is, it would have to be reground and polished for the new curvature and so it would have been essentially just a mirror blank.
[1] https://articles.adsabs.harvard.edu/pdf/1988BAAS...20..366.