No, the pixel density is noticeably different.
My 30” ACD is 101 ppi.
My 2010 27” iMac is 109 ppi.
And my 2017 27” iMac is 218 ppi.
Text is noticeably larger on the ACD, and that larger text size is more comfortable to look at IMO.
As mentioned, to achieve the same equivalent pixel density as the 30” ACD, the non-Retina 2560x1440 iMac would need to be 29.2”. Similarly, a Retina 5120x2880 screen would need to be 29.2” to achieve the same text size. A Retina 5120x3200 screen with that text sizing would be 30” of course.
Conversely, a 5120x3200 screen at the same 218 ppi pixel density as the 27” 5K iMac would be 27.75” (not 30”).
I own that 2010 27” iMac, 2017 27” 5K iMac, and 30” ACD and thus have been able to compare them directly side by side with the same versions of macOS.*
*I can use the 2010 iMac in Target Display Mode so I know what recent versions of macOS look like on that 15 year old iMac. I even know what iPadOS 26 looks like on that 2010 27” iMac.
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