Let me ground it a bit:
He's saying that someone could come along and advertise 0.0.0.0/1 and
128.0.0.0/1 and by doing so they'd hijack every unrouted address block
regardless of the block's ROA.
RPKI is unable to address this attack vector.
Section 4
A ROA with a subject of AS 0 (AS 0 ROA) is an attestation by the
holder of a prefix that the prefix described in the ROA, and any more
specific prefix, should not be used in a routing context.
The route validation procedure, described in Section 2, will provide
a "valid" outcome if any ROA matches the address prefix and origin
AS, even if other valid ROAs would provide an "invalid" validation
outcome if used in isolation. Consequently, an AS 0 ROA has a lower
relative preference than any other ROA that has a routable AS as its
subject. This allows a prefix holder to use an AS 0 ROA to declare a
default condition that any route that is equal to or more specific
than the prefix to be considered "invalid", while also allowing other
concurrently issued ROAs to describe valid origination authorizations
for more specific prefixes.
By convention, an AS 0 ROA should have a maxLength value of 32 for
IPv4 addresses and a maxlength value of 128 for IPv6 addresses;
although, in terms of route validation, the same outcome would be
achieved with any valid maxLength value, or even if the maxLength
element were to be omitted from the ROA.
A property constructed AS 0 ROA for 1.2.4/22 ( in Amir's scenario ) would cause an RPKI participating router to properly mark any more specific announcement inside 1.2.4/22 as INVALID, which is in fact 'addressing the attack vector, WITH the assertion that all routers in the routing domain are RPKI enabled, and discarding RPKI INVALIDs.
The fact that RPKI INVALID routes *cannot* be summarily discarded TODAY because of the state of the internet as a whole is a separate issue.
Fundamentally, in the scenario described by Amir originally, the operator is being dumb by NOT announcing AT LEAST the prefix for their allocation to ensure that nobody can just toss out an announcement to snipe the unannounced space.