Something technically true is actually, really true or correct but it may not be the way people think about it. For example, although people call a tomato a vegetable, technically it’s a fruit.
A birth father may technically be your father — according to a DNA test — but if you've lived with a stepfather your whole life, he’s your dad. Things that are technically true fulfill some exact requirement. Technically, a swing set might be in your neighbor’s yard, but since their kids are grown up, they consider it yours.