They are probably referring to the upright version of pi. In the picture below, you can find on the left an italics version and on the right an upright one.

If you're using LaTeX, you can obtain the upright pi with the upgreek package and the command \uppi, see this Q&A on TeX SE.

They are referring to the upright version of pi. In the picture below, you can find on the left an italics version and on the right an upright one.

If you're using LaTeX, you can obtain the upright pi with the upgreek package and the command \uppi, see this Q&A on TeX SE.

More details on this convention can be found in the IUPAC Green book, which follows the ISO/IEC 80000 standard. From p. 7 of the linked document:

The overall rule is that symbols representing physical quantities or variables are italic, but symbols representing units, mathematical constants, or labels, are roman.

And from p. 8:

The symbols π (3.141 592. . .), e (2.718 281. . . , base of natural logarithms), i (square root of minus one), etc. are always roman [...]