That post has some fundamental errors regarding how double-spending works (explained in a comment there).

Thanks for your explanation. I'll copy it here if you don't mind so people don't overlook it.

There are lots of errors in your post. I tried to explain in

https://bitcoil.co.il/Doublespend.pdf how double-spending works.

In order to double-spend you don't need to find 6 blocks in a row. While the network is finding the 6 confirmations you're also mining so you'll be less than 6 blocks behind - and even then you don't need to find blocks in the row, you just need to catch up (e.g., if you're 3 blocks behind, you win if you find 2 blocks, the honest network finds another one, then you find 2 more). If you have 51% of the hashrate your chance to successfully double-spend is 100%.

On the other hand, in order to successfully double-spend you need to attempt to double spend. You actually have to commence an attack and there are costs involved. So referring to the chance to obtain a "streak" in a given time period is pointless - you have to relate the cost of an attack, its potential gain and its success chance.

Re terminology - a "51% attack" (or more accurately a ">50% attack") refers specifically to when you have majority hashrate and thus guaranteed success. If you have less than 50% it's not a >50% attack, it's just normal hashrate-based double-spending which can either succeed or fail.

Anyway, it's well known that for <50% hashrate, success chance decreases exponentially with confirmations and thus 24 is much more secure than 6. It is also well-known that Litecoin obtains 24 confirmations as fast as Bitcoin obtains 6.