Not sure why the article is talking about 1000mph, that target/claim was dropped long ago and pretty sure you'll find no mention of it on their website.
Look harder. Literally the second FAQ entry.
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Q: Are you still planning to attempt 1,000mph?
Yes, but there are now two phases. The first phase is to break the world land speed record – currently 763.035mph (1227.985km/h). This is necessary to understand how the car behaves as it initially enters the transonic and then supersonic speed levels. Part of this phase will be high speed testing in excess of 400mph (643km/h), which will take place at Hakskeenpan in South Africa. The second phase is to target the maximum design speed, which is around 1,000mph (1,609km/h). This will be dependent on the success and review of the first phase.
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This is hardly a surprise. Testing is always incremental - validate the basic chassis to 200+mph on a UK runway, then they went to South Africa and went to 628mph (the design intention being to engage the rocket motor at 650mph), so they've basically validated the initial jet-only phase and the desert wheels (as opposed to the rubber tyres used for runway testing). The final stage being then to go back with the rocket, break the LSR and then press on to 1000mph.
Doing it in two hits is to be expected. When JCB set the diesel-powered LSR they broke the record but ran again to hit their design speed of 350mph (because the existing record was rather unambitious, so it was inevitable they would break it on a proving run as they built up to the design speed).