Is archietect certificate considered as completion certificate??

Yes. In this regard clarification was issued.

5. Supply of services The following shall be treated as supply of services, namely:—

(a) renting of immovable property;

(b) construction of a complex, building, civil structure or a part thereof, including a complex or building intended for sale to a buyer, wholly or partly, except where the entire consideration has been received after issuance of completion certificate, where required, by the competent authority or after its first occupation, whichever is earlier.

Explanation.—For the purposes of this clause—

(1) the expression “competent authority” means the Government or any authority authorised to issue completion certificate under any law for the time being in force and in case of non-requirement of such certificate from such authority, from any of the following, namely:—

(i) an architect registered with the Council of Architecture constituted under the Architects Act, 1972; or

(ii) a chartered engineer registered with the Institution of Engineers (India); or

(iii) a licensed surveyor of the respective local body of the city or town or village or development or planning authority;

Refer: schedule-ii-cgst-act

Explanation to the said entry clarifies that in case of non requirement of the said certificate by the competent authority then any of the following certificates need to be considered....

Wherever there is non issue of completion certificate from local authority, or its non mandatory.