I'm not having a laugh, and you clearly don't follow tennis in any way shape or form. All the names you mentioned can be considered greats with great sporting achievements under their belts. However .....

She won the US open as a qualifyer. The first qualifier, man or woman, to capture a Grand Slam title ever!

She won the US open without dropping a single set.

She won a grand slam in a debut season on the WTA Tour.

She won the US open as an 18 year old.

Those 4 stats should do the job, tbh the top one alone is enough

I believe almost all of the Women's top players on tour this year participated and got knocked out. They must have lost the odd set or more accurately 2 sets to some tennis players nobody has heard of

It's all opinions obviously but to me it's the single greatest sporting achievement by a British sportsperson. The fact is was so unexpected, as you rightly said, illustrates the incredible achievement perfectly.

She was the deserved winner of the SPOTY award , landslide winner would be my guest. Even if she never wins another grand slam nobody can take the above away from her. It's in da books.