Let me suggest a thought experiment to explain why airspeed increases
when entering a thermal.

For simplicity let's assume that we are flying in trimmed, steady,
unaccelerated flight descending at 2 knots and we fly into a sharp
edged area of rising air. That is, we can continue to climb while
flying straight ahead. What happens?

When the wing encounters the thermal, the angle of attack increases
suddenly. This increase creates more lift, provided we're not stalled.
The weight of the glider hasn't increased so Newton's 2nd law dictates
that the glider accelerates upward (g's in the seat of your pants). I
realize that there are other factors. For example, the pitching moment
of the wing changes with a change in AoA. The pitching moment due to
the distance between the CG and the center of pressure of the wing
changes because lift increases with AoA (assuming the wing isn't
stalled). The downwash on the tail changes with the change in
circulation about the wing. Etc. But these changes will be
overshadowed a fraction of a second later because so far only the wing
is in the thermal.

Next the horizontal tail encounters the gust with twice the area of the
vertical tail with the same moment arm. It's also intended to produce
its lift in the downward direction. (This is exactly analogous to a
glider weather vanning into a cross wind on the ramp except that the
vertical tail isn't producing any lift until the crosswind starts and
gravity isn't going to cause the glider to pick up speed on the ramp.)
The same thermal edge that increased the angle of attack on the wing
decreases the AoA (in this scenario horizontal tail AoA is upside down)
on the tail and hence, down force on the tail. So the glider pitches
nose down.

Pitching forward quickly reduces that angle of attack and the upward
acceleration stops. But the glider has been disturbed from its trimmed
combination of airspeed, angle of attack and weight. Assuming that
the stick is not changed, the glider's airspeed will increase. As the
airspeed over the tail increases it produces more down force and the
glider begins to pitch up again and slow down(static stability).

At some point the glider will slow down and start to climb at the rate
of the thermal minus the original 2 knot descent rate and it will be
back in trim.