I believe they can block immediately.
BUT, when you tech a throw, you are in normal jump.
This was actually in the throw mechanics thread that was just posted. It means that you can guardbreak them after they tech your throw.
e.g. Cable AC to hk airthrow… if they tech, Cable actually gets falling j.hp, \ /, AHVB…
In Corner, a teched Mag throw leads to / , lk,hk, ad d/f, d+lk<mk option select guardbreak to ROM or whatever corner combo you want.
So in a way you can get guaranteed hits, but not because you can just combo into it easily as far as I’ve ever seen. These techniques or the most useful with Magneto since it effectively turns a “failed reset” into a new combo. Cable’s is less useful because they can take less damage by just taking the airthrow. Magneto’s resets a combo whether they ate the throw or teched it (corner only, usually).
On my team, IM with Dhalsim AAA assist in corner will go to infinite with approximately the same timing whether they tech or take the throw. On throw, assist hits and IM picks up for infinite. On tech the assist is blocked and IM guard breaks with the pick up for infinite.
Sent in unfly can hp,xx,unfly, GB faling j.lk+sim-aaa,hp, \ /, sim hits, dash, s.hk, / , … on my team. hp combos into unfly falling lk,hp on hit and with the right timing also guardbreaks when the initial hit is blocked. If they pushblock, Dhalsim still often hits as they fall onto him (no more block since they’re in normal jump!) =)
So you can in certain cases get fun, guaranteed stuff when they tech your throw. The concept was initially presented in “Advanced Tactics” by ShadyK and some others some years back, IIRC. It was the same video that first really introduced pushblocking for sake of guard cancelling (instant action once pushblock timer has recovered, allowing you to punish strings and patterns normally unpunishable on block).