AKA they could not have built it themselvs in time. The engine is a huge part of a JET plane so if they cant build that then what worth does the rest have?
The designed chassis is just some metal junk without it. So beeing unable to build the engine, which might not even fit the plane (or need massiv redesigns) is a huge deal.
hence why it took so long because they needed to make sure their native engine actually worked since that was the first plan, once they realized it took too long then they switched to the RD-10 (jumos engine) so if they really wanted to throw caution to the wind they could have strapped the jumos and did a test flight of it by mid june of 1945. for refence it took ~150 days for the shooting star to get a prototype from the start of the design phase with an existing engine provided by the brits
Tl;DR: the slavs could easily have this thing out in mid 45, their stubbornness prevented it