I don't think it's quite right to say that Bitcoin is "backed" by digital energy. That would be like saying gold is "backed" by the cost of diesel fuel and backhoes. Bitcoin and gold are both backed by societal expectation that they should have value. It's more correct (imo) to say that Bitcoin is secured by the energy it takes to write to the blockchain and the proof of work protocol, but that's not what gives it its value.
If the value of one bitcoin was pegged to the value of the electricity and computer hardware it takes/took to mine that coin, mining would never be profitable because money in would be exactly equal to money out. If the value of gold or bitcoin were determined by the cost of mining, then miners would operate continuously regardless of any economic conditions because they would know that they would always be able to get back at least the value of the cost of their mining activities no matter how expensive they were. In reality, it doesn't work that way. The value of gold and bitcoin are set by market conditions. Miners use the price to determine if mining is feasible, market participants don't look at the cost of mining to determine their asking price.
Also - utilizing landfill gas to generate cryptocurrency could only be called carbon negative under limited circumstances. Only in the case where the landfill was simply venting the gas to the atmosphere and not flaring it or running a generator to sell to the grid/power their own facilities (both of these are the normal practices for landfills), then it would be carbon negative. Otherwise, it's either carbon neutral (converting from flaring to a generator that runs miners) or increasing carbon by add load to the power grid and requiring additional generation that wouldn't be used otherwise.
IMO - I think the best description is to say that Bitcoin trades like a commodity (rightfully so, imo) but is not truly a commodity. Part of the definition of a commodity is that it is a physical good. What sets it apart from stocks, bonds, and other investment assets is that owning commodities means you own physical, tangible items, rather than an on paper representation of partial ownership of an abstract idea like a corporation. Hold on to a soybean contract long enough and a truck will show up at your house full of soybeans - you don't have that with bitcoin - only a string of numbers in a secure distributed ledger.