You're correct in saying a guy could light load a 6.5 Creed to Grendel specs, but I'd REALLY question why you'd want to? I personally don't think a guy gains enough by having a reduced velocity, light bullet load, rather than running the 140/147's at full speed. It's not much powder savings - you're talking about ~35grn H4895 vs. ~41grn H4350, whereas you're talking only ~28grn Benchmark or Varget under the Grendel...
To follow: I think it's very popular, albeit also very cliche' to claim a round "loses steam" or "drops like a brick" past XXX yards. It makes me laugh when I see guys regurgitate this kind of "advice," because typically, it's blatantly incorrect, and believing such nonsense stops a lot of guys from ever experiencing fantastic rounds. 600yrds is a warm up for the Grendel - I start at 500yrds whenever I take mine out, and usually end up 800-1000. Most Grendel loads meant for long range shooting will stay supersonic past 1,000, typically somewhere in the 1,100 to 1,200yrd mark. If a guy knows his load, owns a suitable ballistic calculator app, and has a laser rangefinder, the only hard parts are accurately reading the wind and pulling the trigger.
Slower rounds drop more per range than faster rounds. More aerodynamic bullets lose velocity slower than less aerodynamic bullets. The 6.5 Grendel can't run as fast and can't handle bullets as heavy and aerodynamic as the Creedmoor case. Does that mean the Grendel can't be used at long range? Nope. Does it mean the Creedmoor has a more long-range-friendly trajectory? Yup. But that flatter trajectory comes at a cost - I run more powder and lower barrel life in 6.5 Creedmoor, and considerably more recoil. What I get for that price is less drop and wind drift.
While it's not a 6.5 creed vs. Grendel, here's an example of "Creed vs. Grendel" from a couple weeks ago. Below is a side by side comparison in impact between a 6mm Creedmoor and 6.5 Grendel, at 875yrds (NOTE: The 3 on the bottom of the 6 Creed target were sighters, first time having it at this range, followed by a 0.2mil elevation correction, then the group). Target is 12"x20" 66% IPSC. Note - the 6 Creed load is a light load, running 2916fps with an SD of 6 and ES of 24 with the 105 Berger Hybrid, while the 6.5 Grendel is factory Hornady Black with an ES of 78 and the 123 ELD-M, pushed to 2562 avg. The relative splatter size is indicative of how hard we're hitting, and the plate swing response and audible "DING" are equally different between the two. Both of these rifles are sub-moa at 100yrds, somewhere 0.4-0.7MOA on most outings from a bipod and bag, prone - but you can see how the ES starts contributing more vertical dispersion into the group. My job as the shooter was more difficult with the Grendel, because I had to pick my shots more closely to match the selected wind condition, but you can see, despite considerably more drift, I didn't have much more drift dispersion with the Grendel than the Creed. It's not impossible to manage, just easier with a faster, more aerodynamic round.
The 6.5 Grendel is a fun little round, and I happen to really LOVE the idea of a 6mm AR/Turbo/FatRat to get even more range out of the x39/Grendel case. I've used Grendel AR's for a few PRS type and "Sniperggedon" type long range matches (prone troop, hit to advance to next 100yrds). I readily admit, running a 6.5 Creedmoor or 6 creed, or even a 6 dasher puts me at an advantage over the Grendel, but the Grendel can hang - it just puts more burden on the shooter. If I miss a 1MOA target out to 800-1000yrds with either of my Grendel AR's, it's because I pooched it, not any fault of the rifle or cartridge.