I always view such claims with a very big grain of salt.
CPU's aren't the big power hogs any more (Unless you are gaming) And what I find what typically happens is they increase the screen size/brightness/resolution, or decrease the battery size (mAh) to save cost. I haven't seen runtime budge much in recent years due to these factors.
States that only 1 will reach that 18hrs - depends on the specs. probably one with a dim screen and very large battery pack.
I have an Ice Lake i7-1065G7 Dell 13 2-in-1 (1920x1200, 16GB,1TB) and I get 12 -13 hours of use - the previous gen Dell 13 I had before would get me about 9 hours. This is not a DTR and for my use case, the 12 hours is more than enough - never that far away from an AC plug. The new Dell is much speedier than the old Dell - noticeably faster.
IF Dell releases a Ryzen 4000 based 13/15 and can compare against a Dell 13/15 with Cascade Lake and Ice Lake - then we can see how they stack up