The YouTube app doesn't even let me choose video quality settings when I'm on cellular data. So it's probably a YouTube app limitation.
If you want to see if your videos are being throttled, just do a speed test using Fast.com. That uses Netflix servers - if BingeOn/DataMaximizer is on, it'll be throttled to 1.5 Mbps.
Carriers I've used: Sprint, T-Mobile, Cricket, AT&T, Verizon, Page Plus, Virgin Mobile, Movistar, AT&T Mexico, Telcel, Straight Talk, RingPlus.
Phones I've used: iPhone 6S, iPhone 5c, HTC One M9, Samsung Galaxy S3, LG G2, BlackBerry Storm, Nokia 820, Motorola Atrix HD, iPhone 4S, HTC One V, iPhone 4, Nokia Lumia 900, Nokia N9, Samsung Focus, Samsung Captivate, Nokia N900, HTC 8525, iPhone 3GS, iPhone, Nokia E61i, Siemens A56...
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