No, just because the software is hosted on SourceForge does not mean that it is open source. In the United States (and many other countries), works are protected by copyright and the author must grant specific licenses. A work that is uploaded to SourceForge without a specific license granted is protected by copyright.
However, unlike GitHub, SourceForge requires that source code uploaded is released under an OSI-Approved License. They ask users who encounter a project that is not clearly released under an OSI-Approved license to report that project (likely so it can be removed until it comes into compliance with the terms of service).
If you find a project on SourceForge that does not have an explicit license, you should not treat it as open source, but "all rights reserved". It's the safest thing to do. You can't assume a given open source license, since there are drastic differences between MIT, GPL, and LGPL as three examples.
When you submit, post, upload or otherwise provide Code to
SourceForge.net, you must designate promptly the software license
pursuant to which licensees, including Slashdot Media, obtain rights
with respect to such Code. Except as otherwise expressly permitted by
these Terms, any Code submitted to SourceForge.net must be licensed to
Slashdot Media and other licensees under a license that is: compliant
with the Open Source Initiative (“OSI”)’s Open Source Definition
(http://www.opensource.org/docs/osd) or certified as an “OSI-Approved
License” (http://opensource.org/licenses). Please note that Slashdot
Media is not affiliated with the OSI.
From the SourceForge FAQ:
Is the software you provide free?
The software hosted on our site is both free and Open Source. Each project is expected to provides their own Open Source License which contains any restrictions on the use of the software or code. If a project is not free, does not provide source code, or does not contain a valid Open Source License: please report it here.