the user could not evaluate the equial parse tree created manually

The placeholder in the projection just happens to have type 101h (same as identity) but internally they are different

q)type(parse"+[;2]")1
101h
q)type(parse"+[::;2]")1
101h
q)
q)null(parse"+[::;2]")1
1b
q)null(parse"+[;2]")1
0b

In your case of a single elided item, I would resolve the projection using value or apply & then evaluate the parse tree

q)value((@;;(),`date;(+;1));`t)
@
`t
,`date
(+;1)
q)
q)(@).((@;;(),`date;(+;1));`t)
@
`t
,`date
(+;1)