It is easy to fake timestamps, you just have your software write in a number into a block of the fake chain it is creating. It is difficult/impossible to fool existing nodes into believing the network is valid. However, an independent node (of the network) sees two equally valid histories based on the rules of the network. There is no way it can independently verify whether a timestamp was forged, it's just an integer in a block. This is also the case for Bitcoin, but the cost of creating that timestamp is governed by the PoW difficulty rather than a free digital signature given an attacker with ~50% of the network stake. And the attack can continue free of charge, whereas with Bitcoin you must keep expending resources to keep up with PoW because the most difficult chain wins.

It's a difficult attack to be sure because owning that much stake in a network is unlikely - but it is absolutely not impossible because many PoS systems especially have very lopsided distributions. Losing the ability for new nodes to know what is the "one, true chain" without needing outside information is a problem. How big of a problem is a matter for debate, but it can't just be brushed off as so unlikely as to be impossible.