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No. 25% is the increase of the interest on the BTC balance. Normally you get a daily interest corresponding to an annual interest rate which is 4.08%. If you keep 500000 FUN after one year the annual interest rate will be 5.10% (because 4.08x1.25=5.10).
This isn't one of the big selling points unless you intend to keep a good amount of BTC for several years on the site. With compounded growth it could grow to be a nice amount if you're willing to wait.
Honestly, seems more risky than not. I've done some really rough math and the break even point on the different tiers of FUN are pretty long, and that's assuming it holds value consistently, which alts as a rule just don't do for a prolonged period of time. Could this possibly be an exception? Maybe, but this all seems a little too pumpy to my liking.