For a post-1700 profile, a link, like the one in Lisa's question, to an entry in an Ancestry tree meets minimum WikiTree sourcing standards, though many of us would like to see other and stronger sourcing. A link just to "Ancestry.com" does not meet the minimum standards.
See https://www.wikitree.com/wiki/Help:Sources_FAQ which includes an explanation of why sourcing guidance is much more relaxed for post-1700 profiles. The policy on this has frequently been discussed in G2G.
Because WikiTree is collaborative, we can all help to improve sourcing for profiles where it is weak.
Particularly for profiles created in the early years of WikiTree, there are quite a lot of broken links to entries in Ancestry family trees. This will often be because the family trees have been deleted since.
I should perhaps add that there are a lot of pre-1700 and pre-1500 profiles that were created before WikiTree tightened its policies on sourcing requirements for early profiles and that have inadequate sourcing, sometimes just to an Ancestry tree, which does not meet current sourcing expectations. This is a big legacy problem. Some of us spend a considerable part of our WikiTree time improving such profiles, but it is a laborious task likely to continue beyond my lifetime.