I think, legally speaking, it is clear that the person is not eligible for the Recovery Rebate Credit. 26 USC 6428(d) (as well as the corresponding section from 26 USC 6428A for the second round recently added) says:
For purposes of this section, the term "eligible individual" means any
individual other than-
(2) any individual with respect to whom a deduction under section 151
is allowable to another taxpayer for a taxable year beginning in the
calendar year in which the individual's taxable year begins, and
Note that it says "allowable". 26 USC 151(c) "allows" exemption deductions (which has been zeroed for 2018 through 2025) for dependents:
(a) Allowance of deductions
In the case of an individual, the exemptions provided by this section
shall be allowed as deductions in computing taxable income.
(c) Additional exemption for dependents
An exemption of the exemption amount for each individual who is a
dependent (as defined in section 152) of the taxpayer for the taxable
And 26 USC 152 defines "dependent" as someone who meets certain conditions as a qualifying child or qualifying relative, without regards to whether they are actually claimed as a dependent by someone (i.e. regardless of whether someone took the section 151 exemption deduction for them) or not.
So in your example, the person meets the tests to be a dependent of his parents, so he is a "dependent" of his parent as defined in 26 USC 152. 26 USC 151 "allows" his parents to take an exemption deduction (which is currently 0) for him. Regardless of whether the parents choose to actually take an exemption deduction for him (by claiming him as their dependent), this still does not change the fact that they are allowed to take the deduction. Since the deduction is "allowable" to the parents for him, the person is not an "eligible individual" for the Recovery Rebate Credit.
In order for this person to claim the Recovery Rebate Credit on his 2020 form 1040 line 30, he would have to fill out the Recovery Rebate Credit Worksheet in the instructions for line 30. The first question in the worksheet is "Can you be claimed as a dependent on another person's 2020 return?" with answers Yes and No. In order for him to claim the credit, he would have to answer No. I don't think he can honestly answer No if he meets the conditions to be claimed as a dependent, even if nobody actually claims him.