The article notes how a court transcript from last week "shows that Mueller’s team was deeply concerned that Manafort – after he had pleaded guilty and agreed to cooperate with them – continued to lie in hopes of receiving a pardon."
For his part, Trump has claimed that anyone who flips and testifies against him, his campaign, or his administration is lying to help the prosecution. Thus, he said rhetorically about a pardon "why would I take it off the table?"
One has to imagine there would be a political firestorm should Trump attempt to pardon Manafort after he's sentenced on March 5. He would lose support from some Republicans in Congress and hand a hot-button issue to any and all 2020 election opponents.
As well, he could potentially face obstruction of justice charges either via a sealed indictment or an indictment after he leaves office for dangling a pardon in front of Manafort so overtly. An otherwise legal act can be considered obstruction of justice if done in a way to impede an investigation or prosecution.
With that said, Trump could just let Manafort twist in the wind after he's sentenced and never pardon him. He's well known for demanding absolute loyalty but then showing no loyalty to anyone who does.
And a postscript here for those who don't already know: "keeping schtum" is British slang for keeping silent about something.