Whether coconut or bamboo charcoal is used has no effect on the quality of the fibre. It's marketing hype. All the differences between the 'bamboo' & 'coconut' textiles & any other bit of cloth are down to the processing, not the raw materials.
Coconut husks are sold for fuel, for charcoal making, for turning into coir for matting, & for numerous other uses. There's a ready market for 'em. I don't know anything about the bamboo charcoal market, but I see no reason why one shouldn't be able to tap into standard Chinese industrial feedstock sources, as with bamboo-based viscose pulp. For the viscose, I think it'll just be a question of which pulp mill one buys from, at which time of year.
Anyone thinking it is significant is being conned. The charcoal isn't going to have any magical qualities, & the viscose pulp will be just like any other. Bamboo cellulose, wood cellulose - all cellulose. And once it's been steeped in caustic soda, shredded, aged, & mixed with carbon disulphide, any chance you might have had of telling the difference has gone.
The firms that I've seen claiming "70% bamboo" specifically refer to bamboo fibre, which means viscose. This is very, very polluting stuff. Look up how it's made. As for the "bamboo charcoal yarn" - I'd like to know what 'bamboo charcoal yarn' is. I can't think of any way of making a t-shirt out of 70% charcoal. BTW, the Gelert website says that t-shirt is 50% polyester.
I will actively avoid buying from any firm that claims special properties for these materials because of the plants used as charcoal or cellulose sources. Their marketing people are either insane or crooked.