Is there any evidence for that claim (other than some big miners saying so)?
Surely some miners are making such statements, and you should already realize that you do NOT need a lot of coins in order to manipulate prices downward. If someone has 10k coins, then s/he it can accomplish a lot of BTC price manipulation.
Miners dumping is NOT a logical conclusion,
In short, the answer is "no, there is no evidence that miners are holding".
There is Jorge, and you know it:
http://www.asx.com.au/asxpdf/20141020/pdf/42t0mqrygyzd74.pdf
Despite the figures, the company is downplaying the effect of short-term price fluctuations on its business and still maintains its performance is "robust".
Unlike most bitcoin companies, Digital CC Limited is a public company, trading on the Australian Securities Exchange (ASX) as 'digitalBTC'. This makes it is the only major mining operation with an obligation to provide such information, giving outsiders a rare glimpse into the inner working of a bitcoin mining company.