To quote from the very article you link to, according to critics/oppn the timing seems related to PiS' eroding popularity at a time of economic difficulties, as well apparently another way to criticize Berlin for not doing enough for Ukraine.
The ruling Law and Justice (PiS) party has repeated calls for compensation several times since it took power in 2015, but Poland hasn't officially demanded reparations. [...]
The combative stance towards Germany, often used by PiS to mobilize its constituency, has strained relations with Berlin. It intensified after Russia invaded Ukraine amid criticism of Berlin's dependence on Russian gas and its slowness in helping Kyiv.
Donald Tusk, leader of Poland's biggest opposition party Civic Platform, said on Thursday that Kaczynski's announcement was "not about reparations".
"It's about an internal political campaign to rebuild support for the ruling party," he said.
PiS is still leading in most opinion polls but its edge over Civic Platform has narrowed in recent months amid criticism of its handling of surging inflation and an economic slowdown.
According to another article in Politico this is still not an official request by the gov't of Poland.
Kaczyński’s demand is still not official government policy, and Warsaw hasn’t issued any formal request to Berlin.
Otherwise it goes over the same points, adding that Germany is extremely unlikely to pay as they rejected a similar call from Greece, apparently considering all debts settled by the 1990 treaty that reunified Germany.
One might guess that some EU-level disagreements about Poland's internal policies (rule-of-law etc.) might also have something to do with PiS' combative attitude against Germany on this, in general. On the latter issue, as of beginning of Aug, the EU was still not releasing "€35 billion in loans and grants from its pandemic recovery fund" to Poland, over those disputes. And as a matter of timing...
The [Polish] parliamentary report [...] was commissioned five years ago but [its] publication was repeatedly delayed. [...]
In a keynote speech in Prague earlier this week, the German chancellor, Olaf Scholz, had signalled a more confrontational stance over Poland’s crackdown on judicial independence, saying EU funds should be tied to the maintenance of rule-of-law standards.
Of course, PiS' would probably not agree with characterizations like the above.