Why does the opening of an epoxide occur via an Sn2 like mechanism when using a methanoate ion as a nucleophile and methanol as a solvent? Below is a picture of the mechanism:


I understand the mechanism but what I don't understand is why it's $\mathrm{S_N2}$. The solvent used is polar and protic, there is a tertiary carbon that would be suitable for $\mathrm{S_N1}$ as well. For what reason does this reaction occur exclusively via $\mathrm{S_N2}$?
Quotation from Clayden: 


  Without protonation, the epoxide oxygen is a poor leaving group, and leaves only if ‘pushed’
  by a strong nucleophile: the reaction becomes pure SN2. Steric hindrance becomes the controlling
  factor and methoxide attacks only the primary end of the epoxide.