It may well be an excuse but TBH there's a slim chance they might have been telling the truth but not explaining it very well.
Whilst not knowing exactly how Ryanair do their trim and balance it's quite possible they might do what many other airlines do - use what are known as assumed weights ( numbers agreed with the regulator) for male/female/children when doing their load and balance calculations...
That allows the people who do the load and dispatch to work out from the number of male/females/children in each seat row where the aircraft's centre of gravity, or balance point is positioned, which is something it's important to know from a loading and flying POV.
The balance point must not be to far forwards or to far aft so if it turns out when the numbers are crunched the centre of gravity is very near one of the limits then you can't have (if any) swapping between seat rows.
Why? Because if you're very close but inside a loading limit and then there's a significant move in weight- e.g. 3 adult blokes swap from row 1 and go to row 25 and 3 children switch with them and go the other way there's just a chance, in extremis, you might then move the centre of gravity outside the limits.
So long story short, you can't rule the explanation out completely, it's a rare situation but it is credible, and I know it's something that has happened on flights at airlines other than Ryanair.
Bit of a long post but HTH.