Meaning of simultaneity in special relativity
The relativity of simultaneity refers to a pair of spatially-separated events.
The collision event of two objects meeting is not spatially-separated. All frames of reference will mark that collision with a single event.
UPDATE (to address the comment by the OP):
A spacetime diagram is useful to clarify the ideas.
Furthermore, with my diagram, you can see that [from a geometric point of view],
"absolute simultaneity" (that if Alice says $t_P=t_Q$, then all observers say $t'_P=t'_Q$) is really an exceptional case (rather then the rule).
Using my "spacetime diagrammer" https://www.desmos.com/calculator/dha8izuxz1 ,
with event $P(x,t)=(0.5,1)$,
- Alice (in RED) says that distant event P is simultaneous with tick-1 on her clock... (because Alice assigned the same t-coordinate to those events),
- however, Bob (in GREEN) says that distant event P happened before "tick-1 on Alice's clock".... (because Bob assigned a smaller t-coordinate to P than to tick-1-on-Alice's-clock).
The "lines of simultaneity (i.e, equal-assignments-of-t)" are determined by the tangents to the "circles" [in the respective geometries], which is a hyperbola in Special Relativity (arising from the invariance of the speed of light).
The "circle" is determined by the set of all observer "tick-1"s.
The "circle" defines perpendicularity: the tangent is perpendicular to the radius...
"the Space[line] is Perpendicular to the Time[line] (worldline)", as Minkowski described in his "Space and Time".
- For the Galilean case, open the "Metric" folder and slide E to 0. Note how "absolute simultaneity" is obtained since the tangent lines through Alice's-tick-1 and Bob's-tick-1 coincide.
- Proceed further to E= -1 to obtain Euclidean geometry. Note how the tangent lines do not coincide, as in Minkowskian geometry (special relativity).
- Thus, the Galilean case is the exception. But since this is our "everyday intuition" (since we don't rely on super-accurate-clocks [except for GPS] or regularly interact with particles at high-relative-speeds), it's a layer of common sense we have to let go in order to understand relativity.
