I think this is true, essentially all digital cinemas operate on 4K, and it's the common DCP master resolution. *
That said, I have been rather blown away how good a well mastered Blu-Ray holds up on a Barco cinema projector. Kieslowski's Dekalog (Criterion) looks absolutely stunning despite being a humble Blu-Ray.
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(*) DCP cinema masters are typically
4096 x 2160 px vs 3840 x 2160 so the resolution difference is negligible.
Bitrate / chroma / colour depth on DCP is higher though, 250mbps / 4:4:4 / 12bit vs 60-100mbps / 4:4:2 / 10bit on UHD Blu-Ray and 120mbps / 4:4:2 / 10bit on Kaleidescape.
Technically there is no reason why HEVC (UHD Blu-Ray & Kaleidescape) couldn't support DCP tier quality, however with current disc size limits it could be only done on Kaleidescape, which is download/HDD based.