This isn't allowed. The person on the other forum is right.
Your employer can't put down that you are working 24 hours to make you entitled to benefit, that is fraud. It is also fraud for you to continue to claim when you are not working 24 hours.
You are entitled to tax credits in the months that you do work 24 hours. You say that you've worked one overtime day this year, so you'll have been entitled to claim in whatever month that fell in - the other two you were not, because you hadn't worked enough hours.
It isn't worked out on averages, but even if it was, your average isn't currently 24 hours.
Realistically, you might get away with it for a while, because if both you and your employer are telling the same lie it is harder to pick up on. If you or your employer was subject to a random check, though, this would be easy to spot, and you would then be reassessed. Tax credits would likely request evidence of your working hours every week since you started with your employer, and you could end up with a hefty overpayment.
Really, you need to phone up and tell them that you are on a 23 hour a week contract but work 24 hours when you can. They will stop your claim, and ask when that change started. They will reassess from that date.
For your own peace of mind, it might be worth working out how long this has been the case, and calculating the overpayment yourself. I would prepare to have to pay that back, because while you might get away with it for a while, even years, it's rare that somebody gets away with it forever and DWP debts are never statute barred - they may not be able to take you to court after six years, but they will reclaim from future benefits. Come up with a payment plan, however low, and they will likely accept it provided that it is all that you can afford, because you were honest and admitted to the debt rather than waiting to be found out.