You are absolutely right, some people don't know that's why they are selling their Bitcoin hoping for it to dip for them to buy again not knowing that it is possible for Bitcoin to skyrocket or pump from this point and then dip maybe the previous point it pump from or it may not even dip to that point. Even if someone invested in Bitcoin some months ago before Bitcoin take this uptrend one shouldn't sell all his or her coin perhaps they should just sell some and still leave some and later continue there accumulation and holding so that it won't be like they are starting afresh because starting afresh is always hard and it requires a lot of determination and of course consistency to grow back again.
An investor who started his investment few months ago has nothing to do with selling part or all of his bitcoin, unless the person is just chasing after the few bucks and that's not encouraging at all for someone who has the mindset of long term hold, or call himself an investor. Even if the person starts his accumulation journey again at some point as you are saying, the person will still sell off, if there is also a future little gains, outside of his maturity date. If it happens that few months or one year later and there is another upward trends, the person will also sell and that's not how investor should behave. An investor who started his investment journey few months ago, has no business with selling not even one Satoshi of his bitcoin that has been accumulated. The focus should only be buying and should not be tempted with any little profits seen so far. People who thinks like that, or behaves that way are purely traders and they shouldn't confuse themselves as investors and people should not also refer to them as investors. And I don't think it's a good advice to give to someone in a thread like this, so that we don't confuse newbies who might be vulnerable to such an unhealthy advice.