Suction doesn't really exist. Higher pressure in one side than the other does exist though, and sometimes we refer it to suction.
Those suction valves, honestly I don't know how they work, exactly.
"Closed by pumps pressure in normal operation, they allow the engines to be fed by gravity if the inner tank pumps fail"
I believe, or my guess is, that the suction valves are closed whenever the pumps are on, because of the suction these create. Then, in the absence of pump pressure when the pumps fail, there is no more suction and the valves will open, thus allowing the feeding of the engine by gravity. Does that make sense? any engineer or someone with first hand knowledge of these suction valves?
As I understand it, fuel cannot just flow through the pump by gravity. It needs an alternate path to the engine feed line, which would be via the suction pumps.
So, according to the above, only gravity pushes the fuel into the engines. Not suction. More precisely, weight does push the fuel. Gravity is a field, but what pushes the fuel is a force, or a pressure in the line, if you like.
Gravity feeds the fuel into the engine, and the more the weight of fuel above the valve, the more the pressure in the line. if gravity shifts away and some fuel goes to the other side, it doesn't exert its weight on the fuel covering the valve any longer, and therefore there is less pressure on that point, now. That could lead to a flame out.
Even if the pumps do not uncover, which I agree with you: they will not uncover in any conditions the pumps would not.