They are arguing that the EU currently has protections in place for workers rights, which could be lost in the event of Brexit.Their 2017 manifesto said they would accept the result of the 2016 referendum. They clearly haven't. They've been arguing that workers' rights and other things would no longer be protected by the EU, which is a lame excuse to delay Brexit, as it should be a thing for the country's government to deal with. Nothing would be stopping them from getting into power and protecting those things.
It's not a lame excuse for delay. It's a very good reason for delay and as the current opposition they are seeking to make sure that the current government doesn't drop workers rights below the current EU regulation.
I still dont see how this is at odds with their manifesto in 2017.