Can we rely on SelfAdjointEigenSolver returning orthogonal eigenvectors?

I use Stan, which in turn uses Eigen internally.

I need to factorise a real symmetric matrix A as QDQ’. Since A is symmetric, the eigendecomposition can return such factorisation (but it may in principle return a different one TDT⁻¹ such that TT’≠I). The documentation for the Eigen function SelfAdjointEigenSolver used by Stan does not mention whether the eigenvectors returned are orthogonal (or, equivalently, that the operation coincides with the SVD). It looks like that function internally uses the symmetric QR algorithm that, as far as I know, does return the factorisation I want. Testing with a few random matrices seems to confirm that the eigenvectors returned are indeed orthogonal.

Since the fact that the eigenvectors are orthogonal is not documented explicitly, can we rely on this behaviour or not? If so, should it be also written in the documentation?

Thank you in advance.