Timestamps are not reliable since they are user reported. This is a well known problem of distributed networks where computers come on and offline and have no reliable way to validate them.

Bitcoin intentionally avoids trusting the timestamps as much as it can, and although they are used in some aspects of the system, they are used in aggregate (using averages) and there are safeguards to avoid calculations being skewed by maliciously reported timestamps.

If someone were to attack the network, they would have to consider the timestamps they use, but considering they report the timestamps themselves, it is not a factor that helps in any way against 51% attacks.