I don't think it has anything to do with the Cheltenham incident.
His queries seem to have been more about what Carruthers is like to play against.
I can hardly see somebody like Fleck saying "He's a good player gaffer but I can't play alongside somebody who once did a wee in a pint pot".
Actual Wilder quotes from an unnamed source:-
“We’ve spoken to other pro’s about Samir, including lads at our place, about what they think,” the United manager said. “And they’ve all told us he is a quality player, very tough to compete against. Samir is a quality performer and, as far as we’re concerned, there’s even more to come from him. He ticks all the boxes for what we want here at this football club, including the right character, desire and attitude.”
“Samir was coming towards the end of his deal down there and, arguably, could have waited until it ran out to put himself in a better position money-wise,” Wilder continued. “But that’s not something he was interested in doing which tells you all you need to know. Samir made it clear he wanted to come and that he was excited about the prospect of representing this club, of playing at Bramall Lane.”
“Samir is someone we’ve been tracking for quite a while,” Wilder said. “Sometimes, you bring people in to make an immediate impact and there’s nothing wrong with that. Samir can make an impact because of his ability but he’s also someone for the long-term who we think can grow with this football club.”