It’s not speculation. You can google “federal prison menu” and see the national menu that prisons supply. Here is the 2022 menu. You will note as you read that menu that there are obviously vegan food items that SBF could eat from every single meal of every single day of the week. Breakfast? Fruit, coffee, bread, branflakes… Lunch? Beans, sweet potatoes, mash, salad, rice, baked potato… Dinner? Tacos, salads, tofu, soups, tater tots, cornbread, corn on the cob, hummus… In most cases he even has a viable main option, and even if he can’t he could always trade his main to someone else for a side of theirs. Not to mention stuff he can buy in the commissary - ramen noodles, candy, crackers, cookies etc.
So in summary, SBF is lying and trying to drum up sympathy for his own self-inflicted situation. I’m sure prison food sucks compared to what mommy makes or what his ill gotten fortunes could buy, but he is not reduced to bread and water.