Well, to give them the benefit of the doubt: A) if you really did just take the front mech off a working HardRock and then fit new tyres, saddle and cables you'd probably be under-selling it at £70, and B) it clearly states they have fitted "new cables, a new shifter, a new chain and a new front chainring and bottom bracket too … oh and new brake levers! The bike has a new seatpost and a new seat, new handlebars, a new stem and new grips too!" as well as the tyres. I don't think its a bargain, its not what I'd do to it but they are (admittedly weirdly) cool at the minute and look at the alternative new bike for £350 - yuck!
Anyway, this one is well over £1000:
https://www.ebay.co.uk/itm/166699220807 and there are an additional two more similar conversions on eBay for more money than this one, and perhaps even worse is this deeply odd full refurb resto-mod of a 2002 HardRock at £600:
https://www.ebay.co.uk/itm/176978143030
TLDR: People think HardRocks are great and some people obviously buy them for an amount of money the RB community would balk at!