That's right, you didn't get it wrong. No matter how right you make the nether portal, if it's the light block occupies inside of the nether portal then you can't light it until you remove the block.
You can mess up your friend with this trick. 🙂
It's work in 1.17 until this snapshot/version
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100% intended, the portal specifically needs air; any non-air block, any at all, will prevent the portal from activating. Additionally, a light block can output a higher light level than nether portals do, effectively removing light.
It's also a command-only block for custom maps, they're not supposed to be removable without creative mode, just like command blocks, barriers and structure blocks.