He only pitched 122 innings at LSU last season and then like 6 more in the minors after the draft, so less than 130 innings for the year. Prior to last season the most he ever pitched was 85 innings. So, I am sure they have a ceiling in mind for his total innings this year across all levels, like maybe 175 innings or so. They don't want him to waste innings in the minors (while they wait our roster rules sure) so they would build his innings slowly in the minors and then when he is stretched out, he will be up.
It is a no-brainer to leave him down the first few weeks to get the extra year of control. The only question is if they are also going to try to wait out the Super 2 clock as well, which would take them well into June. Everything about the Pirates history suggest they will wait out the Super 2 clock, but they have done some things differently so we'll see. I still can't believe they started Jones in the majors, potentially missing out on the extra year of control for him.
Even Stephen Strasburg made like 11 AAA starts before being called up and he was the highest graded college pitcher ever drafted, even higher than Skenes. So it's not ridiculous for Skenes to make 10 starts or so and get fully stretched out in AAA before coming up. He only went 3 1/3 in his third start so he need at least 3 more starts to get up to the 6-7 inning range and he won't be called up before he can go 6-7.