You’re correct that the answer is no. See: Types · The Julia Language
Your first type has a type parameter with the name Float64 which does absolutely nothing. The fields x and y can still be of any type. Moreover, the fact that the type parameter has a name which is the same as an existing type (Float64) is totally irrelevant.
Your second type, on the other hand, actually restricts the x and y fields to be of type Float64.
To implement a parametric Point type, you want something like this:
mutable struct Point{T}
x::T
y::T
end
which creates a struct with a parameter named T and two fields which must both be of that same type T.