What does your POH say about opening spoilers at high speed? Below
Maneuvering Speed (80 kts, I checked) it shouldn't be a big deal.
FWIW, I think you did the right thing in losing speed right now, rather
than thinking of the FAA implications. Could the feds ground you from
flying your ultralight anyway (probably)? ;-)

Forgive me if this strays from the thread a little, but I think it is
worth expanding on the relationships between maneuvering speed (Va),
spoilers and speeds/loads. Here is my understanding:

1. There is no relationship between Va (maneuvering speed) and
spoilers, except that Va is determined with spoilers retracted. Va is
based on pitch (elevator inputs). There is a general belief that
below Va you can do anything with any flight control, and the glider
cannot be damaged. This is may be true in some, perhaps many,
situations, but it is not true in any certification sense. [See EASA
glider regs CS 22.335]

2. Once spoilers are deployed, the loads for which the glider is
certified drop to +3.5 from +5.3 (utility) / +7.0 (aerobatic). [See
EASA glider regs CS 22.345]

For the reason stated in #2 above, "The Handbook of Glider
Aerobatics" (Mallinson and Woollard, 1999, page 30) states "It is
nearly always better to slow a glider by 'pulling g' rather than by
operating the airbrakes". They are speaking here of aerobatic gliders
(rated to 7 g's) during aerobatic maneuvers. There is some difference
in the loads/slowing that can be achieved by utility gliders. Also,
there are other considerations when using 'g' to slow (symmetric
loads, etc.). For the purposes of this discussion, I think I'm safe
with the summarization that the choice of spoiler over 'g' should not
be automatic in all situations.

This is not to say that spoilers may or may not have been an
appropriate response in the case being discussed, or many other
situations. Rather, I want to dissuade those who might feel that
spoilers are an appropriate response in every situation. Certainly
deploying spoilers and pulling high g's could be a catastrophic
combination.

I am certainly open to correction if there is an error in my analysis.

Regards,
Eric