The problem isn't with allowing one person to sit on their own in a park, it's that if you allow one person to do it, you can't then justify saying no to the next person, or the person after that and so on until the park is full. And we also don't know enough about the virus to know how dangerous it would then be if an hour later someone else came along and sat in the same spot - is the grass contaminated? Literally no one knows, so why not all do our park to minimise risks? Although as you say, that is almost immediately contradicted by allowing DIY shops to stay open!
I read about a study by an American university where they invited people who thought they'd had the virus to come along and be tested and found only 40% positive results. Some of that was down to concerns over the antibody test they were using maybe not being able to detect antibodies until an unknown amount of time after, or the antibodies not building up immediately, but they still concluded that a significant number of people who thought they'd already had it in fact hadn't. Don't forget, there will still the the usual selection of seasonal flu's and colds, some of which will be very similar to a mild coronovirus case but won't generate the correct antibodies.
A lot of the people who turned up to be tested had already assumed they were immune, even though the presence of antibodies doesn't necessarily imply that. As you say, that's why testing is so important. If you think you've had it and are immune, it's natural for your behavior to change, but people who think they're immune and aren't will end up putting themselves and others at risk.
