According to ISO/IEC 27001:2022, clause 6.1.3, the Statement of Applicability (SoA) is a document that identifies the controls that are applicable to the organization's ISMS and explains why they are selected or not. The SoA is based on the results of the risk assessment and risk treatment, which are the previous steps in the risk management process. Therefore, the justification for the exclusion of a control should be based on the risk assessment results and the risk treatment plan, and should reflect the purpose and objective of the control.
Control 5.18 of ISO/IEC 27001:2022 is about access rights to information and other associated assets, which should be provisioned, reviewed, modified and removed in accordance with the organization's topic-specific policy on and rules for access control. The purpose of this control is to prevent unauthorized access to, modification of, and destruction of information assets. Therefore, the justification for the exclusion of this control should explain why the organization does not need to implement this control to protect its information assets from unauthorized access.
The justification given by the organization in the question is not acceptable, because it does not reflect the purpose of control 5.18. An access control reader at the main entrance of the building is a physical security measure, which is related to control 5.15 of ISO/IEC 27001:2022, not control 5.18. Control 5.18 is about logical access rights to information systems and services, which are not addressed by the access control reader. Therefore, the organization should either provide a valid justification for the exclusion of control 5.18, or include it in the SoA and implement it according to the risk assessment and risk treatment results.