If I install an extension that sends agrees to a website then yes, it's the same as using a cat as a proxy, because objectively the request for consent has been fulfilled without irregularities. It's even mentioned in sentence 2 of Recital 32, using technical measures to auto-consent is valid as long as it is done clearly.

However uMatrix does not send agrees; in fact, it shreds most 'requests to consent' to pieces before relaying the page to the user as a side-effect of blocking third-party cookies+scripts. So unlike the cat proxy, uMatrix will never send a (deceitful) agreement back and the website owner never gets a reply for his inquiry.

All a website owner gets from a uMatrix user is the wrench they threw into the consent acquiring procedure, and I doubt that's enough to signify consent as defined in GDPR.

GDPR Art. 4 (11) https://gdpr-info.eu/art-4-gdpr/

  ‘consent’ of the data subject means any freely given,
  specific, informed and unambiguous indication of the data
  subject’s wishes by which he or she, by a statement or by a
  clear affirmative action, signifies agreement to the
  processing of personal data relating to him or her;