Hi, welcome aboard. I'm afraid I'm not familiar with the story of Cornelis Smakman and neither with any Boer prisoners being sentenced to be shot at Elandslaagte.
I've searched the database of the Anglo-Boer War Museum in Bloemfontein for the name 'Smakman' as POW and as a foreign volunteer and can find nobody matching that name. (
www.anglo-boer.co.za/database-search/
)
It does seem strange that if he was shot (executed) that a coup de grace wasn't delivered and that neither was he buried but just left there. The battle continued into early evening so one would wonder if there was time to convene any sort of hearing or if it was a case of being summarily shot.
There was an abuse of a white flag by the Boers at Elandslaagte and British troops going forward to accept the surrender were fired upon. This seemed to have resulted in some taking a "take no prisoners" attitude following the incident but then again several prisoners were indeed taken at Elandslaagte and not shot.
When the cavalry cut into the retreating Boers there was a bloodbath. Boers screaming for mercy and trying to surrender were cut down. Many wounded from both sides spent the night lying out on the battlefield and were only attended to the next day. It would be no stretch of the imagination to imagine that Smakman may have been shot down while trying to surrender and spent the night on the battlefield wounded and was only found the next day and was then taken prisoner.
There were many instances of Boers being shot. Especially later in the war, Boer prisoners were routinely shot for the wearing of British uniforms. Boer prisoners were shot for using dum-dum ammunition. Rebels, ie Cape and Natal colonial citizens caught fighting for the Boers were shot. In some instances there were hearings. In other instances 'guilty' prisoners were summarily shot. Boers were shot for train-wrecking and other such offences. The shooting of Boer prisoners has always been highly controversial and remains so to this day. The Breaker Morant incident was in the news again recently.
Cheers,
Mark