Why would a 2019-nCoV protein sequence in the NCBI database match a protein submitted in 2018?
2019-nCoV is a virus that originated from the bat (at least this is the current hypothesis). It shows 96% squence similarity to the BatCoV RaTG13 sequence (see reference 1), showing its origin.

It still is 87,99% identical to the "Bat SARS-like coronavirus", which explains the hit you found and is not unexpected, as these viruses are very closely related (see reference 2). 

These viruses are closely related, so I wouldn't expect too many differences at all. Then envelope proteins can be critical for function/structure of the virus, so mutations there might occur less frequent. And if they occur, I would only expect few changes over time, so with this little time gone, probably no mutation is seen yet. Additionally, due to the redundancy caused by the codon degeneracy, not every mutation in the genomic material translates into changes in the protein.

References:


Full-genome evolutionary analysis of the novel corona virus
(2019-nCoV) rejects the hypothesis of emergence as a result of a
recent recombination event
Genomic characterisation and epidemiology of 2019 novel coronavirus:
implications for virus origins and receptor binding