Keep in mind that they never claimed that the MDEK1001 had a range of 250 meters. They said that the DW1000 has a range of up to 250 meters.
The MDEK hardware probably won’t be able to reach 250 m but should be able to get a lot closer to it with different settings. Remember this is a dev kit, its job is to show some of the more common use cases and applications and then act as known good hardware for your own firmware development. It’s not intended as an end product or to be the best possible hardware implementation for any specific application.
I’ve not used the MDEK dev kit myself but I believe the firmware source code is available and so it should be possible to change all of the radio settings if you want to. And ultimately all of the hardware details are available and so it is possible to write completely custom firmware for the board if you wish to.
Details of what the possible radio settings are and their impacts on speed and range are to be found in the DW1000 user manual and the related application notes.
As a basic example on our system with things set to the maximum speed we would get a range of around 40 meters. Changing the data rate from 6800 kb/s to 850 kb/s and making no other changes got us to 60 m range but forced us to drop the measurement rate by 20% due to the longer transmission time. Increasing the preamble and SFD would have added more range but we didn’t need it and didn’t want to have to drop the speed any further. For the maximum range you would need to drop down to 110kb/s and increase the preamble significantly.
Which channel you use also has an impact, the higher the frequency the higher the free space path loss.
The other simple but somewhat dubious method to get more range is to turn the transmit power up. This will put you over of the FCC/CE/Other regulatory bodies maximum transmit power limits but will give you more range. The exact legal situation doing this depends a lot on where you are but most places allow you to be a little more relaxed about the limits during product development and evaluation as long as 1) you stop if there is any indication you are causing interference and 2) you’re not somewhere that this could be considered dangerous like on a plane or in a hospital. Obviously not an option for long term use or a commercial product but may be an option while trying to work out if your basic concept will work.