Sure, they have some things (arguably low-value, but you may disagree) on GitHub, but they do not have the main protonmail service - where I could run my own Protonmail server - on GitHub.
Setting the bar awfully high there, aren't you? Very few companies open source everything, and I'd say that it's pretty reasonable to say you support open source and open standards despite that.
If instead they said "Protonmail supports open source" then I would totally agree with you, but I think my conclusion is fair to draw from their language: "Protonmail will always be open source."