Contractor is correct...just-poured concrete takes, IIRC, 3 to seven days to attain 50 percent compression strength, 28 days to reach ~90+ percent full bearing ability.
Properly cured concrete can take thousands of pounds of compression, but has almost no ability to bend without cracking, hence reinforcement to stiffen, keep cracked slabs together.
Would be VERY careful connecting south yard rails to shop floor ones until some time passes.
Rules of architecture/engineering/contracting I've learned in school and on the job...Everything settles, concrete cracks.

Hope floor pour is going well.