This might be slightly off-topic, but I want to note that even the cancellation of a Certificate of Citizenship does not imply revocation of US citizenship. A Certificate of Citizenship is issued in many cases as proof of citizenship for someone who is already a citizen (who automatically became a US citizen, either by birth abroad to a US citizen parent, or as a minor permanent resident living in the US with a US citizen parent). Since the grant of the certificate was not a grant of citizenship, the cancellation of the certificate is not a revocation of citizenship. This is mentioned in this USCIS manual page:
However, such a person may have an additional basis upon which to
claim automatic acquisition of citizenship. Accordingly, if that
person’s Certificate of Citizenship is cancelled by USCIS, but the
person subsequently provides evidence that he or she automatically
acquired citizenship through some other basis, the cancellation of the
first Certificate of Citizenship does not affect the new citizenship
As well as the Board of Immigration Appeals ruling in Matter of Falodun, which ruled that judicial proceedings to revoke naturalization were unnecessary in the case of cancellation of a Certificate of Citizenship, because the certificate did not grant citizenship, and cancellation of the certificate did not change the person's citizenship status.
A certificate of citizenship only provides documentation of United
States citizenship for persons who claim to have obtained that status
derivatively. See 8 C.F.R. §§ 341.1, 341.2(c) (2016). It does not
confer United States citizenship but only furnishes recognition and
evidence that the applicant has previously obtained such status
derivatively, that is, upon the naturalization of a parent or parents.
See Section 341(a) of the Act. Thus, the issuance of a certificate of
citizenship, like a United States passport, only serves as indicia of
citizenship. It is not a grant of United States citizenship.
Administrative proceedings to cancel a certificate of citizenship
under section 342 of the Act, which provides statutory authority to
cancel a certificate of citizenship that was illegally or fraudulently
obtained, are different from denaturalization proceedings under
section 340 of the Act. The main difference between cancellation and
revocation proceedings is that cancellation only affects the document,
not the person’s underlying citizenship status.
The exact same logic should apply to the revocation of a US passport -- like the Certificate of Citizenship of the person in the case above, a US passport is only granted to someone who was already a US citizen (or US national) -- the grant of a US passport is not a grant of US citizenship (or US nationality) itself. Therefore, the revocation of a US passport only affects the document and does not affect the person's underlying citizenship status. A person whose US passport is revoked, who believes that they are nevertheless a US citizen, can still seek to claim the rights of citizenship, including entering the US at a land border. If the government disagrees and puts them in removal proceedings, their US citizenship can be adjudicated in those proceedings and subsequent appeals.
There are several reasons under which the US government is allowed to deny or revoke a person's US passport, without any question regarding their US citizenship or nationality. This includes issues such as unpaid child support and seriously delinquent tax debt. It also includes foreign policy or national security reasons, as upheld by the US Supreme Court case of Haig v. Agee (1981); in that case, Philip Agee's passport was revoked for national security reasons, which has similarities to Edward Snowden's case, but Agee was never regarded as having lost US citizenship.
This archive of 7 FAM 1380 mentions that in the case of people denied US passports or had US passports revoked for various reasons like unpaid child support, fugitives in extradition, etc., they can still be issued a limited emergency passport with an endorsement that limits it to direct return to the US. I believe that if Edward Snowden were to seek to return to the US, any US consulate would happily issue him such a direct-return emergency US passport.