Not sure NASA thinks the SLS is actually required.
The article quotes Bridenstine saying this:
"Bridenstine was clear that SLS would be used for EM-2, describing the monster booster as "a critical piece of what the USA needs to build" in order to put the large objects into space."
I've not found official NASA statements on what it's going to do with Orion, but there's this:
which says things like:
"NASA plans to launch elements of the Gateway on the agency’s Space Launch System or commercial rockets for assembly in space." and "NASA plans to resupply the Gateway through commercial cargo missions" and "NASA will leverage capabilities and plans of commercial satellite companies to build the next generation of all electric spacecraft."
All through the main body of the statement, NASA refers to commercial rocket launches, commercial space craft development, and so on - even though the "update" at the top states: "Together with the Space Launch System (SLS) and Orion, the Gateway is central to advancing and sustaining human space exploration goals"
The funny thing is, that update is dated after the main body of the statement. It looks like after the original statement was released (which heavily emphasises NASA's commitment to using commercial space technology), someone decided that NASA had to make it LOOK like it really, really wanted the SLS - even though NASA's actual plans are rather ambivalent on what heavy launch vehicles it might actually end up using.