The non-exported shallow is supposed to operate on data.frame arguments since 1.14.2. But:

> df=data.frame(a=1:10)
> attributes(data.table:::shallow(df))
$names
[1] "a"

$class
[1] "data.frame"

$row.names
 [1]  1  2  3  4  5  6  7  8  9 10

$.internal.selfref
<pointer: 0x56250b748b10>

It appears setselfref is called regardless of if the input is indeed a data.table.

Also, test 1984.27 was supposed to catch it - but the way data frames/tables comparison is coded in test currently it doesn't. The culprit are these lines in the comparison:

        setattr(xc,".internal.selfref",NULL)   # test 2212
        setattr(yc,".internal.selfref",NULL)

# Output of sessionInfo()
R4.0.5, data.table 1.14.3