czy zapis w postaci async="true" zadziała tak samo jak wpisanie w tagu samego "async"
W praktyce, pewnie tak.
Technicznie, to to nie jest poprawny atrybut.
The async and defer attributes are boolean attributes
The presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value. If the attribute is present, its value must either be the empty string or a value that is an ASCII case-insensitive match for the attribute's canonical name (...) The values "true" and "false" are not allowed on boolean attributes.