My new 2007 5.3 3.73 Avalanche had the same requirement.
I followed it because I had to make a 4400 mile roundtrip to pick up our new Roo 23SS.
Personally I wouldn't tow without doing the break in period first.
But the 50mph thing is weird. I seem to remember that it was 50mph during the 500 mile break in period.
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Dan-Retired California Firefighter/EMT
Shawn-Musician/Entrepreneur/Wine Expert
and Zoe the Wonder Dog(R.I.P.) 
2016 PrimeTime TracerAIR 255, pushing a 2014 Ford F150 SCREW XTR 4x4 3.5 Ecoboost w/Max Tow Package
Equalizer WDH
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