The term "sell out" differs from person to person. I despise the term "True metal" to describe the genre of certain bands, but of course in your post, it is used under a different context. I would say every band evolves, but there are some who actually do something that they are not comfortable with or change their musical direction not because it's something that happens naturally or they are just experimenting, but do it because all the other popular/successful bands are doing it even though their genres might be different. In other words, they move away from what music they play just to follow a certain trend. I would term that as "sell out". There are also bands that stick to what they believe in and create the kind of music they enjoy irrespective of whether it brings them more album sales, popularity or fans and irrespective of the current trend. I would classify these bands as being non sell outs or true metal in this case( I find the use of the term "true metal" pretty appropriate in this scenario
).