To understand some of this, you need to take a historical perspective of Juniper/Junos. Originally Junos was written for L3 operation only. Later L2 support was added, and with it the need to support things like VLANs (Bridge Domians) and tagging, IRB, etc. Some of this support came via flexable-vlan-tagging syntax.
Later Juniper introduced EX switches which were L2 based, with L3 capabilities, so VLANs and access/trunk (tagged) ports were standard syntax. This syntax differed from orginal Junos M/MX CLI syntax. These switches did NOT support concept or syntax of flexable-vlan-tagging - this was M/MX/PTX only.
With the introduction of next-gen EX models (those that now run ELS) Juniper standardized on a single CLI which favored original M/MX style, with slight modifications. So now all of the newer platforms, EX9200 and EX4300 being 2 of them, support the original M/MX syntax/CLI. The flexable-vlan-tagging support is on all ELS platforms, not just EX9200/EX4300.
Hopefully this makes some sense as to 2nd doc statement.