Hydrolox engines have very high ISP, but lack thrust at liftoff. Due to the low density of hydrogen, they require huge tanks that are as wide as possible, to reduce surface area and thus boil off.
SRBs work well with wide first stages as that affords strong mounting points. And they provide significant thrust for liftoff.
Yeah I wonder how much "tying down" the test required, or if the engine alone is insufficient to make it fly (but still, if you make it too light it might wobble too much, etc).
The SRBs are certainly fire and forget but I'm not sure how the ratio of SRBs/Main engine thrust are compared with something like the Space Shuttle
SRBs work well with wide first stages as that affords strong mounting points. And they provide significant thrust for liftoff.