I'll take a stab at this although I haven't been following BB for long and may have part of this wrong:
1. Tiffany has stage 3 localized cancer that has a chance of "cure" by removing all the cancer. Since it's still localized (basically all in one chunk), they could cut it out, get clear margins, and there is a non-zero chance that she would not have cancer anymore. The faster they do this, the less chance it will metastasize to other areas in the abdomen or more distantly which would be stage 4. Tiff has already had months of chemo and is ready for a surgery decision, which seems to be made at this point in favor of doing it.
2. BB has stage 4 cancer with peritoneal metastases (from what I gathered from watching some, but not all of his videos). Since he is Stage 4 but not with distant spread (hopefully), they can treat him with cancer debulking and HIPEC, but it's not likely to be curative. Because of the size and amount of tumors, he is not currently considered a good candidate for this treatment. If he can use chemo to get his tumors shrunk in number and size, he may be a better candidate for it. But it's not like Tiffany's cancer where they think they can surgically get it all. They'd have to just pour chemo in his abdomen and hope it takes out most, if not all, the remaining cancerous areas.