As_xxx are TField's methods that transtype internally. In TField.As_xxx, xxx means, indicates the type of data we provide (when it is an assignment, i.e. aField.As_xxx:= x;). So, the field knows, then, if it should transtype x before its effective storage (in accordance with its true type).
And for the opposite, it's simply asking the TField to transtype its internal type towards an external typed variable. So yes: when asking for the value with AsInteger (your question), the TFloatField should round its native real value towards the nearest integer.