Ill keep this brief
1) Warren and Sanders are competing for the same bloc of liberal voters in the Dem primary, with Biden courting the moderates in his camp. Anyone who has basic understanding of the candidates can observe that.
2) In order for one of them (Sanders or Warren) to be the representative of the liberal faction of the Dem party, one of them has to beat out the other, and do so quickly to have any chance at beating Biden in the second half of the primary season
3) Hillary Clinton, the polar opposite of what the liberal faction likes and in fact rather loathe due to 2016, just heavily criticized Bernie, calling him unlikable, without allies in Congress, etc.
4) Since Hillary is the virtual representation of a democratic centrist career politician, criticism from her is effectively a certification of being a 'radical outsider' which Warren fans and Sanders fans are drawn towards.
With Warren and Sanders competing to be the 'radical outsider' among the liberal base, and with Hillary Clinton herself singling out Sanders for criticism, she has effectively made Sanders THE outsider candidate, which he could use to cement his claim that he is the forebearer of the liberal faction of the party, since the spokesperson (one of them at least) of the center of the party identified him as the person to be aware of.
This, combined with AOC's previous endorsement and Warrens continued slide in numbers, should give Sanders a major boost in cementing his claim as #2 in the Dem primary