As far as davening ‘beyechidus’ being better than davening with the tsibbur without a hat (or even a jacket) this is truly an astonishing opinion and I daresay that something was lost in the translation (as these are not Rav Kanievsky own words)
This sounds very weird. I’m not a big posek, but I am quite certain also that davening with a minyan outweighs the importance of wearing a hat and jacket.
Not so astonishing or weird. As my R”Y put it, it’s a balance between the inyan of tefillah b’tzibbur and the inyan of k’vod malchus shoayim (my R”Y felt that k’vod malchus shomayim is more important, and that, indeed, one should wait until he has a hat even if he will lose out on tefilah b’tzibur).
I’ve seen quoted in R’ Elyashiv’s name (he should have a refuah sheleimah), that tefillah b’tzibur takes precedence over wearing a hat and jacket.
Obviously, if someone lacks appreciation for how a hat and jacket reflects k’vod malchus shomayim, he won’t begin to appreciate that it’s even a shaila.