UT1 is also off from UTC by up to 900 milliseconds. If for example a timestamp is obtained as SELECT datetime('now'); then this is UTC (assuming the clock of the computer, or smartphone is synchronized with a standard configured NTP client). To convert it to UT1 one can
1) download from https://www.iers.org/IERS/EN/Science/EarthRotation/UT1LOD.html the Earth rotation data which include the preliminary and final DUT1 (UT1-UTC) and correct the UTC timestamp for the offset.
2) in the US register at the NIST, https://www.nist.gov/pml/time-and-frequency-division/time-services/ut1-ntp-time-dissemination, and configure the NTP client such that it synchronizes instead to the NIST UT1 NTP server.
Both are quite tedious, I doubt that many Sqlite users are having datetime in "UT1 Zulu time".
The Sqlite documentation is correct:
"Universal Coordinated Time (UTC) is used."
"It is also assumed that every day is exactly 86400 seconds in duration."
This is together is not the same as using UT1