So what makes those drones different from jets? The fact that jets have to take off from somewhere? Is that it?
To a large extent the answer to this question lies only in Putin's head. However, one can reasonably guess at his reactions through studying historic precedent and his public statements. Though a Javelin or an NLAW is termed a defensive weapon, there is no reason it cannot be used offensively. At the end of the day, these weapons supplied by third countries will kill Russian soldiers. Yet still, the supply of these systems does not cross the threshold in Russian minds where it would necessitate requalifying those countries supplying these systems as direct participants in the war. The Bayraktar is a different issue, but I suspect that it is being treated as a subject on a parallel track because it's an existing relationship between Ukraine and the system's Turkey-based manufacturer. Putin and Lavrov still threaten Erdogan with all sorts of consequences for continued shipments.
Now, what do we have with the Mig-29? We have a weapons system that when new costs $30M+ per unit, of which Poland - a country many times richer than Ukraine - only itself has a few dozen. Poland is seemingly willing to completely disarm itself, leaving its own air nominally unprotected, and transfer these to Ukraine - effectively raising Ukraine by many positions in the global list of countries by number of deployed combat jets. The gesture in itself, is several magnitudes more serious than transferring dusty anti-tank missiles.
In practical effect, it would seriously complicate Russian efforts at maintaining air superiority. Thankfully, if one is to be honest with himself, the Mig 29 has near zero value in air-to-ground attack. However, it is still a very fast and extremely maneuverable jet, that will complicate operations for Russian tactical bombers and close air support.
As a final point - I'm sure that the NATO side is aware that at this point the Russians have claimed 100s of the Javelins and NLAWs as war trophies. What is to say that Russians cannot at some point get access to the Polish Migs as well? There are probably many curious innovations present on Polish migs that Russians would be interested in studying (upgraded radars, avionics, communication equipment, targeting equipment, etc). Is NATO comfortable with Russians studying weapon systems that are infinitely more complex than a one meter metal tube with a rocket inside?
PS: AntV is correct, it does not have to be a nuclear strike on Poland. It could be a conventional strike against the shipment when it is in transit. A few dead Polish truck drivers, and soldiers escorting the column. Surely, something everyone can close their eyes to eventually if no further escalation follows. Absolutely for certain they would cease supplying any coal, oil, gas, or refined products to Poland. Probably begin targeting Polish businesses and persons in RU, BY, KZ, AR, TJ.
It could also be a singular demonstration meant to show the rest of the Western alliance by what is meant with "serious consequences". For example, a barrage of cruise missiles against a very specific airfield in Southern Poland.