Been a videoguy since I switched from photography over 3 decades ago.
Low light quality images have been a nightmare since camcorders began to shrink from 2/3 equivalent to 1/2 and now 1/3.
EOS cameras (i.e 700d) claim iso up to 25600 but what does it mean in video in terms of lux?
Will a camera like this perform better in low light than Sony NX5?
I plan to buy one of these for time lapse.
Not sure exactly what you're asking. Modern cameras continue to get better and better in poor lighting conditions. At our shop we have a Canon 7D, 5DIII, and C300. Hands down the C300 is the king of the pack!
As a point of comparison our Canon 5DIII blows away our Canon 7D in low light scenarios. We can shoot at higher ISO on our 5D and still have a usable image where our 7D will become overrun with noise (higher ISO = more noise). Our 7D starts to fall apart around ISO 2500. Our 5D can run close to ISO 10000 and still be usable. The chip technology in a 7D is almost 5 years old. Sensors have advanced quite a bit since 2009 when it was released.
For many of the manufactures claims I disregard how high it can go as a measure of effectiveness. However, typically the higher it goes then the cleaner it is at the lower ISO's where people typically shoot. So instead of ISO 2500 barely being usable on our 7D, our 5D can shoot at ISO 2500 all day and look clean. Again, because the sensor technology has advanced quite a bit.
Another important aspect for you to consider is lensing. Canon L-series lenses are fast and can drastically help in low light settings. That's a double-edged sword as you know. The faster you run the lens the more depth of field you're creating so focusing can be a challenge. But opening up the lens to let more light in can help offset some ISO issues. In terms of lensing I will always give the nod to Canon over Sony.