Not every architecture has add with carry, for example. If by "intrinsics" you mean e.g. ADC then it may very well happen that a non-constant time "fallback" will be faster than constant time implementation. What I try to say is that promising constant time fallbacks creates pressure for native implementations which except for crypto is not desirable, IMO.
The branchless, constant-time versions of Add and Sub from Hacker's Delight were originally proposed, but it was faster to compute the sum normally, and check a condition to compute the carryOut and borrowOut.
Also, if someone wants to double-check the math/bits Add64 benchmark, that would be helpful. If it is literally never executing the 'carryOut = true' case, then it's a bad benchmark. It should do a mix of operations.