I think "speedotann" has made a reasonable assumption that the aircraft landed without clearance, as the original post references an article from AvHerald stating that the aircraft landed without clearance. The aircraft was not NORDO, as it acknowledged a frequency change to the tower frequency and communicated with ground. For whatever reason, they did not communicate with the tower. As there is no mention of an emergency (the only allowable reason to land without clearance) with the ground controller, we can assume that they did not, in fact, land the aircraft within regulations. (Had a light gun been used, the aircraft would have landed with a clearance and there would be no article.)
You suggest a lack of understanding of NORDO procedures, but the document that you yourself direct us to read is very clear:
4.4.3: Landing Clearance
At controlled airports, a pilot must obtain landing clearance prior to landing. Normally, the airport controller will initiate landing clearance without having first received the request from the aircraft; however, should this not occur, the onus remains upon the pilot to request such clearance in sufficient time to accommodate the operating characteristics of the aircraft being flown. NORDO and RONLY aircraft shall be considered as intending to land when they join and conform to the traffic circuit. Landing clearance will normally be given when an aircraft is on final approach. If landing clearance is not received, the pilot shall, except in case of emergency, pull up and make another circuit.
4.4.5: Arrival Procedures – NORDO Aircraft
Before operating into a controlled aerodrome, pilots shall contact the control tower, inform the tower of their intentions and make arrangements for clearance through visual signals.
People make mistakes. There is every indication that these pilots made a mistake, either in the operation of their functional radio, or their improper landing procedure with a failed radio. Yes. They will get a "talking to"
Most importantly, this serves as a good reminder to all of us that 1000 safe landings doesn't make the 1001st automatically safe. Little mistakes can add up to big, bad results. This forum is, among other things, a place to learn from mistakes lest we don't put an aircraft down on top of another aircraft.