Why not? We already kill neghvars with mirandas, and galaxies with b'rels in PVE, how is that any different? Just earlier I killed a borg cube with a K't'inga. This ship power/age/size argument just doesn't fly in this game, and certainly not for restricting what can be in T5. The game doesn't care, why should we?
Blowing up Negh'vars with low-tier ships usually only happens in ... you guessed it, low tiers, where nobody really cares what happens. B'rel-lookalikes have been continually upgraded over the past century and may easily encompass a dozen classes by now. And finally, if you can blow up a Borg Cube in Omega Leonis with a K'tinga - congrats! Only goes to show that low-tier ships are still viable at higher tiers, so there's even less of a "need" for a T5 Constitution.
Yup, the game sure has its flaws when it comes to ship continuity, but that doesn't mean we have to add to them.