They do flex, although I'm loathe to see how far they'll bend before permanently deforming ! They are a lot more bendy than any steel or ally pegs I've ever used, including the ally one I've got in the picture, which generally don't bend significantly at all. Having said that, since they are titanium, they are a lot stiffer than any other material would be if formed into a peg that small.
I thing they'd maybe be OK for the groundsheet, but I'd be less inclined to use it for the guys or fly, which are under much more tension. They are quite short (5") so I think they may work in fairly solid soil if put in at 45°, but they could be hard work to get in since there isn't much to push on. If the upper soil was anything other than very firm, they could easily pull out.
As HM said, the weight of the pegs is a small fraction, so a heavier but still pretty light peg on the guys at a minimum is probably a good idea, if only for a greater sense of confidence.
I'd be worried trying to use these pegs with the tent in the 60mph+ conditions that some of the country is experiencing at the moment, it's a bit gusty in London at the moment, but not excessively so, a lot less than appears to be happening up north.