If this new Siglent SDM4000A DMM is even close to the price of the DMM6500, why would anyone consider it over the Keithley?
The DMMs at this level are intended for accurate, repeatable, precise measurements and the Keithley DMM6500 and Keysight 34461-5A have demonstrated such over many years/decades of doing exactly this, these are the "Benchmark" DMMs in this territory, and relatively new unknown for Siglent.
Unless the SDM4000A becomes available at significantly lower cost, can't see knowledgable folks selecting it over the DMM6500.
Don't misunderstand, we aren't a Siglent basher, nor fanboy either, and have a significant repertoire of Siglent gear, including the SDM3065X which is a nice DMM, but doesn't compare with the DMM6500 nor KSs either, however is lower cost!!
Best,