Why is word match not working?

[rew1nter]

Isn't the behavior supposed to be the same as grep here?

[BurntSushi]

This has nothing to do with the --w/--word-regexp flag. This is due to the fact that grep uses basic regular expressions (BREs) by default. BREs have different escaping rules and different rules around when * is "special" or not. In this case, since it appears the beginning of the regex, it isn't "special" and matched literally. If you use extended regular expressions (EREs) instead, then the * is treated specially:

$ wmctrl -d | grep -E '*'
grep: warning: * at start of expression
0  - DG: 2560x1600  VP: N/A  WA: N/A  1
1  - DG: 2560x1600  VP: N/A  WA: N/A  2
2  - DG: 2560x1600  VP: N/A  WA: N/A  3
3  - DG: 2560x1600  VP: N/A  WA: N/A  4
4  * DG: 2560x1600  VP: N/A  WA: N/A  mail
5  - DG: 2560x1600  VP: N/A  WA: N/A  browser
6  - DG: 2560x1600  VP: N/A  WA: N/A  text
7  - DG: 2560x1600  VP: N/A  WA: N/A  chat
8  - DG: 2560x1600  VP: N/A  WA: N/A  s1
9  - DG: 2560x1600  VP: N/A  WA: N/A  w1
10 - DG: 2560x1600  VP: N/A  WA: N/A  doc
11 - DG: 2560x1600  VP: N/A  WA: N/A  a
12 - DG: 2560x1600  VP: N/A  WA: N/A  k
13 - DG: 2560x1600  VP: N/A  WA: N/A  ecma
14 - DG: 2560x1600  VP: N/A  WA: N/A  fwd
15 - DG: 2560x1600  VP: N/A  WA: N/A  play

In this case, * is probably being treated as being applied to an empty sub-expression and thus matching everything. ripgrep instead recognizes the expression as invalid and rejects it.

Apparently POSIX says that the regex * produces "undefined results," so I guess grep can do whatever it wants here. This is the insanity of POSIX. It defines a miniature language (regexes) to be used by end users, and specifically leaves huge "undefined" holes in the syntax.

supposed to be the same as grep

In the general case, no, it isn't: https://github.com/BurntSushi/ripgrep/blob/master/FAQ.md#posix4ever