Not equal temperament
Equal temperament was not generally used in 1700, when Cristofori invented the piano. The following excerpt comes from Edwin M. Good's Giraffes, Black Dragons, and Other Pianos (2nd ed. [Stanford University Press, 2001]). The main passage is a footnote, but I've included the preceding sentence as it contains interesting and relevant information.
In 1795, Johann Jakob Könnicke of Vienna made ... and experimental six-octave piano with six keyboards tuned so that one could play in all keys with "just" tuning.*
* Tuning systems were a very intricate problem in this period.... A very early Zumpe piano, a square of 1766, has split black keys so that notes can be tuned to both pitches [e.g., F♯/G♭].... Systems of "equal temperament," ... began to be proposed in the seventeenth century, although it was well into the twentieth before the use of equal temperament became fairly general. (pp. 98-99)
(See also below)
Probably some type of meantone temperament
In Bartolomeo Cristofori and the Invention of the Piano (Oxford University Press, 2019), Stewart Pollens suggests, indirectly, the possibility Cristofori might have used fifth-comma meantone temperament. The following passage are relevant:
Cristofori apparently attempted to explain fifth-comma meantone temperament to Scipione Maffei.... Writing in 1666, the theorist Lemmi Rossi indicated that this particular temperament was then in common use, and thus it may have been the tuning that Cristofori preferred. (p. 119)
The notes Maffei made in connection with the writing of the 1711 article ... shed additional light on Cristofori's ideas regarding temperament.... Cristofori ... seems to be describing, or perhaps advocating, fifth-comma meantone temperament. (p. 240)
With regard to "not equal temperament", a passage from the aforementioned "notes Maffei made" is suggestive:
That the violin is the only perfect instrument, because it does not have keys and one finds in the whole perfect harmony, that is the flats and sharps each in its place.... That on the violin, you can transpose where you want in any sort of key, without hearing an unpleasant effect, because it has equal fifths, and all just, and it does not fall into false and bad sound, like in other instruments. (p. 240).
Following this passage is the portion referred to above, in which a meantone tuning, probably fifth-comma, is described.