Interesting. Although from the quoted definition, it would seem that single coins, even if made of gold, aren't supposed to count as "treasure"? On the other hand, strictly speaking, it's not a "single" coin; it's half of a single coin. Hence, treasure!
According to finds.org.uk, "Under the Treasure Act, single precious metal coins are not considered to be Treasure, but single precious metal coins that have been modified into objects can, if older than 300 years old, qualify as Treasure". The intent of this exception seems to center on coins modified into jewelry of some sort. As the news article states, "The coin had been deliberately cut across the centre with the report suggesting the half coin served as either a friendship token or for a ritual function."
The impression I get from the article is that, where solidi were concerned, small change would be less of a factor than with silver or AE. Along with being approximate to a month's pay for a legionary, a solidus was some serious capital. I'd like to know what kind of house you could put a downpayment on with one of those. With that for readily available context (people on this forum have gone into this in a lot more depth, only recently), the speculation about some sort of ritual or quasi-ritual significance makes more intuitive sense than it would otherwise.