No idea about truthfulness or not. But was that a US carrier? It seems that there is much more "law and order" on US based carriers than on many Asian/European ones. I heard Delta pilots saying: stay seated, we will taxi for an hour and if you get up we will lose our spot and have to start over".

It sounds a bit like keeping people seated just for the sake of having them seated. As for the OPs question: my suspicion is that this is a convenient way for UA to get the first round of "service" out of the way before people start walking around...

In my eyes the whole thing is similar to the "do not congregate near the restrooms" voodoo rule: only experienced it on US bound / domestic flights and I think one Emirates pilot put it best "since we are now flying to the US, I have to tell you this: 'Do not congregate close to the lavatories'..." it really sounded like "whatever, but I have to tell you" - which is very close to the "we have to tell you to seat yourself" answer that other FAs give.