You are wrong and this is happening all the time, but you are clearly not carefully watching football matches

Every player can choose any boots they like most and make individual deal, club sponsorship is only for shirts and kits.
You really don't need to embarrass him 😂
examplens's example was not appropriate that's all but it was a good try. If the example was about an individual player sponsorship then it would be more appropriate however individual sponsorship also does not require you to wear their apparels when you are not in official tour or events.
In casual life you are free to use anything you like.
You get the point of the example, not football boots under the contract, but the jerseys are.
Here I will give you a direct example when we are already talking about large sponsorship contracts in sports. 2008, Wimbledon, Novak Djokovic was then under contract with Adidas, but their sneakers were too slippery. Later it came out in Nike, but with the logo covered. Maybe it was all part of the agreed marketing, but he still covered the competitor's logo to avoid being
kicked out of the signature campaign, a violation of the active contract.
https://www.cnbc.com/id/25346774