> Wouldn't such a change have to be made by the same legislature with a similar majority?
Yes. The legislature voted on the additional language and it did not pass.
> The WA constitution doesn't seem to have anything to say on income tax, just a lot about property tax?
Washington state treats income as property, and property taxes must be uniform and flat within a specific property class.
Technically, this law does run afoul of that because having an exemption makes it not uniform, and per the state constitution property tax is capped at a total aggregate levy of 1% per year.
The state will use the argument that this is not a property tax, but an excise tax on the privilege of earning high income from the state's economy. They used a similar argument for the capital gains tax, and won in the WA supreme court.
