Yeah but you're comparing it to spending £500 on lottery tickets, so it's only fair to use a best guess of how much on average is actually gambled on the PB jackpot prizes. It's going to be a wildly inaccurate comparison if you include the average 90% 'gambled' on £25 prizes.
Why? Are you unaware there are other lottery prizes too? You looked at the chances of winning the jackpot, I've given them to you.
In both cases there are other prizes but that isn't what was being compared, you don't get the option to buy a cheaper lottery ticket or premium bond just to try and win the jackpot only.
If you didn't gamble the £500 on premium bond prizes then you'd have £500 you could spend on lottery tickets, in both cases there is a chance you could win a jackpot or other prizes, in the case of premium bonds your chances of winning a jackpot prize is better than the lottery + your overall EV is better too as the entirety of that £500 is the prize fund essentially.
I guess the lottery could be better IF you had an equivalent safe product paying well over 1% interest (probably more like 2%+) and you gambled that on a lottery draw.