In theory you are correct. However, if it was that simple, everyone would do it, and nobody would be found liable to taxation. Seems a rather easy way out for those currently cheating the system.
HMRC don't only go on what you add to a tax return. They have an enormous amount of information, gleaned from many sources. They will have automated processes going on which will determine which accounts need "looking at". Their first instinct may be to assume you HAVE been trading, and they may ask you to prove otherwise. Many individuals will just give up at this point and pay up, whether or not they are liable.
At this stage no humans at HMRC will have been involved. It remains to see what approach will be taken once there is human involvement. Certainly writing "NIL" in your tax return isn't likely to be the end of it.
And don't assume HMRC won't pursue smaller amounts. An immediate large volume of easy "slam-dunk" cases, irrespective of value, would send a message (whether true or not, who knows), that they are serious about this. The bigger, more complicated higher value cases will take much longer, so the immediate impact will be lost.
