Exactly as ACBelMutie said. There were lots of variation, sure a lot used guard, not just because they're good but also because they battleline. If you look at the results from big tournaments, there have been Nagash/guard, Nagash/Deathriders, Katakros/guard, Katakros/crawler, guard/harvester, and many variations of non-named heroes of the same. Now the power is shifting from the subfaction back to the individual units and well, motrek guard are the top so expect to see them even more than you had before.
If you weren't seeing Deathrider Kavalos Lances in Petrifex in your meta, then don't expect to start seeing them in Staliarch lists now as they were far better before. If people wanted to play that, they could have. Now less so, since they don't have the innate power of PE to proper up the weakness of the build as Deathriders are squishier than Guard and a lot harder to heal.