Well that's a fair point, there are lots of ways to frig this, even down to using a dirty disk, but without knowing what the set up is it's a rather harsh accusation to make. There are plenty of ways to remove such variables, and there is always the possibility of honesty, however difficult you find that to acknowledge.
What's more, even then it depends on the nature of the observed difference - if it's just colour saturation or contrast then clearly (even on a single screen) that could be due to the setup for differences, but if it's blockiness or break up (in e.g. fast moving images) then I'd suggest it could indeed be cable induced - or at least I'd be asking the question what else could be causing it.
A more interesting question might be - how bad is the cheap cable. If it's only HDMI 1.0 and they're passing 1080p across it, then clearly it's not specified to support that so it wouldn't be a like for like comparison. But again I don't know the setup and I can't go. So I'm going to have to rely on others to pass fair and properly informed comment, rather than ill-informed speculation that's verging on libel. (Again.)