Not of boxers

Boxers are on the list of breeds of dogs prone to bloat, but they are 11th (and a very looongg way behind the most at-risk breeds). Of course, that doesn't mean you should ignore the possibility. You should also be aware that IF your dog suffers bloat, it is very likely to be fatal (so knowing the risk factors and signs of bloat is very wise). But at the same time, you would be severely misled if you thought bloat was the near-certainty that is suggested.
I am a little pertubed by the site you posted though. Having quoted research by Purdue University into bloat, it then goes on to state that exercise before/after meals is a risk factor and should be avoided. In fact, the Purdue research (multiple studies) found that exercise is NOT a factor increasing the risk of bloat, exposing that one as a myth (in case it's not clear, the research found that it makes not one iota of difference either way). If you're interested in the actual findings of that research, there is a good discussion in this thread:
http://www.boxerworld.com/forums/dog-health-issues-questions/67610-bloat-risk-factors.html as well as some links to the research itself.