The problem with the above two statements is that they are trying to mix Text mode and Math mode in LaTeX. How do I make symbols like nabla or delta as bold symbols. Boldsymbol and mathbf make bold face symbols and pmb makes very bold face symbols.
As it is virtually impossible to list all the symbols ever used in mathematics only those symbols which occur often in mathematics or mathematics education are included.
In the above only is printed in bold. By providing uniformity over numerous mathematical articles and books these. This will only work if there exists a bold version of the symbol in the current font. To bold lowercase Greek or other symbols use the boldsymbol command.