I don't think that is true. Intellectual property has value associated with it, and if it hasn't been transferred/compensated then they have some implied ownership.
Specific clauses of a vesting agreement, however, are not implied or 'owed' by default.
That's not true because every standard employee agreement in VC backed companies (and in most other ones) also contains an IP assignment. So if you're going to claim that you would have signed a hypothetical employee agreement, it also would have come with a hypothetical IP assignment.
