Yes, Cox... did I ever mention that I stink at names? 😅
Yes, 250lbs is small for a 4-3 edge (i.e. DE). No Clay (nor Nolan, nor Reddick) is not the same as a 3-4 edge (i.e. OLB). They play different gaps. Reddick moved to the Jets and there are a lot of similar questions about him playing DE for them. Likewise for Leonard Floyd and the 49ers.
They might have more gap responsibilities, but they are also farther out. Far fewer snaps against a G and more snaps against the TE/WR/RB on the outside.
It isn't impossible for him to play 250lbs as a DE, but he pretty much has to be in obvious passing downs only unless he ends up being very "special" at defending the run. You tend to pick the "special" players as your examples, but Cox is an UDFA that hasn't shown much special to this point.
As you say, for the 5th guy in the rotation, that isn't as much as a concern. I wasn't comfortable at #4 and expecting him to put in significant NFL snaps this year. My original point was relief that Enagbare was back, pushing Cox and similarly small UDFA backups down to 5th+ on the depth chart instead of 4th. Frankly, unless Cox really busts out this year, I wouldn't be surprised if the Pack went with 4 DEs on the 53 man and put Cox on the PS again.