`highway=` classification is determined between junctions. So it can't directly change to another class in the middle. 
And again, what is a "main line" can be argued. I can say Shun Long Rd is a very large interchange. Or you can see how long is Yuen Shin Rd, or  Tsing Sha Hwy on Tsing Yi. Ma On Shan Rd and Tsing Yi North Coastal Rd may be claimed as entrance / exits as well. 
Therefore to avoid such debate, `=motorway_link` is exceptionally used to indicate special problems. 