Those are probably largely people that probably did just as much activity somewhere other than their home neighborhood (physical-but-nonessential labor, gym, athletic hobbies that rely on now-closed businesses, etc.)
Also, heart disease and type-2 diabetes are years in the making, and a month of moderate exercise and better eating won't make up for it overnight. Not to mention added stress.
When people go back to work, they will have less time at their hands. They will have to adjust to commuting again, taking kids to/from school again, being somewhere at time again.
All of that will cut into their exercise time. All the people who are exercising out of boredom now will stop exercising out of being tired of doing lot of things again.