Index creation on partitioned table is not completely offline in Standard Edition

We have SQL Server 2019 Standard Edition > Microsoft SQL Server 2019 (RTM-CU10) (KB5001090) - 15.0.4123.1 (X64) > Mar 22 2021 18:10:24 Copyright (C) 2019 Microsoft Corporation > Standard Edition (64-bit) on Windows Server 2016 Datacenter 10.0 > (Build 14393: ) (Hypervisor) where index creation is completely offline operation. So every CREATE INDEX statement render the table unavailable for reading/writing. I partitioned one table and to my big surprise when I was creating all the nonclustered indexes on the same partition scheme this table was perfectly readable. Is it a documented feature and where can I read about it? All data modifications of this table was still blocked but there was no blocked readers at all. This database has RCSI enabled if it could matter.