Comment by flohofwoe

I may be wrong but I think a ramjet doesn't need Mach 5 to ignite, Mach 3 to 6 is just where it is most efficient at high altitudes.

E.g. early German experiments during WW2 based on the Lorin tube (https://en.wikipedia.org/wiki/Ren%C3%A9_Lorin) only had to get to 320 km/h to start working:

https://en.wikipedia.org/wiki/Kronach_Lorin