I'm aware of when Mallon made his statement, but you're taking Varadkar out of context all the same.
His actual words:
"Watching Sinn Féin, it is like politics for slow learners sometimes. I think it was Seamus Mallon who famously described the Good Friday Agreement as Sunningdale for slow learners because Sinn Féin finally accepted democracy and peace and power sharing and consent.
"And it does seem that they're finally starting to accept that we shall be a member of the European Union, that we should have a Special Criminal Court."
He means Provisional Sinn Féin are infamous for taking a very long time to come around to democratic norms in the Western world. That's because Provisional Sinn Féin and Provisional IRA always saw themselves as the Provisional government of the whole island. They rejected realities.
Historically, Provisional Sinn Féin and Provisional IRA viewed the EU as an "imperial project Ireland must reject". That explains their rejection of the Special Criminal Court too.
Looks to me like Varadkar is bring taken out of context and Provisional Sinn Féin are trying to exploit that as well.
Political correctness makes little sense when someone is also being taken out of context. It's clear its NOT a reference to people with learning difficulties.
It's a reference to Provisional Sinn Féin long rejecting democratic norms only to accept them well after everybody else has.