As I explained already (see quote below), the coin age time employed to threshold the delay for signing in the variant of PoS you are using, isn't delayed by PoW computation delay. The coin age delay is a fabrication of the UXTO at that point in time. Since the attacker can construct a UXTO from his own stake and since in PoS there is no PoW computational delay impeding the attacker from rebuilding a Long Range chain attack, then the only way to prevent such an attack (i.e. the nothing-at-stake problem) in PoS is to employ checkpoints. This is is known to every expert who has studied PoS.
What is "PoW computation delay" in this context. If you are building a side chain you can assign whatever arbitrary timestamp you want to a PoW block, so there is no required delay. On the true chain, yes there will be delay because it is an honest chain. But on a fraudulent chain, you can do assign whatever timestamp you want, build the next block ten seconds or whatever later, all the while holding the chain privately. Or am I missing an important aspect, I could definitely be and would appreciate you expanding on that if you could.
Edit: Or maybe is the argument that... yes you could build a side chain that could confuse nodes up to a certain time, but that side chain will never catch up to the work added to the main chain?