Odd article on absolutely nothing--where the heck did these guys find this (non-existant) "tradition"?
Pure rubbish, I'm afraid.
Edit: did a bit of checking on the Internet:
2007 Wimbledon final--Nadal (lower-ranked than Federer) sat on the left
2008 Wimbledon final--ditto
So it looks like Wimbledon, the seat of tradition in tennis, doesn't know of this dubious "tradition". Big surprise here...