I think reason why higher concentrate of antifreeze is offered here as faster warmup tip is because of specific heat capaciti is lower when antifreeze concentration is higher (Not because of poorer heat transfer).
Specific Heat Capacity of Water-Antifreeze Solutions
What is normal concentration? I dont know but lets take it is 50%
Specific Heat Capacity unit here is (J / g / ° C)
50% antifreeze mix at 4C: 3.33
65% antifreeze mix (dont have 70% on that table) at 4C: 3.02
So it should warm up about 10% faster